Ascetical theology refers to the systematic study or presentation of spiritual teachings derived from the Christian Bible—including the Hebrew Bible and New Testament—and writings from the Church Fathers. Its purpose is to provide guidance for individuals seeking to follow the teachings of Jesus and achieve Christian perfection.^{[citation needed]} Christian asceticism is commonly thought to imply self-denial for a spiritual purpose. The term ascetical theology is used primarily in Roman Catholic theology. Eastern Orthodox Christian theology carries its own distinct terms and definitions; other Christian movements conceive of God and Jesus differently from both Orthodox Christianity and Catholicism.

The word ascetic comes from the Greek ἄσκησις (askesis), meaning practice. The English term ascesis means "the practice of self-discipline".

Ascetics, as a branch of theology, can be defined as the scientific examination of Christian asceticism. It is characterized as the theological "science of the spiritual life," positioned apart from both dogma and morality. This field is grounded in the truths of faith and aims toward Christian perfection, considered a logical outcome of dogma, particularly the fundamental doctrine of the Incarnation of Jesus. It serves as a resource for both religious individuals and the lay apostolate. Asceticism, taken in its literal signification, means a 'polishing', 'smoothing', or 'refining'. The Greeks used the word to designate the exercises of athletes, developing the powers dormant in the body and training it to its full natural beauty. The end for which these gymnastic exercises were undertaken was the laurel wreath bestowed on the victor in the public games. The life of the Christian is, as Jesus is recorded teaching in Matthew 11:12, a struggle for the kingdom of heaven. To give his readers an object-lesson of this spiritual battle and moral endeavour, Paul the Apostle used the illustration of the Greek pentathlon in 1 Corinthians 9:24. The exercises are intended to develop and strengthen Christians' moral stamina, while their aim is Christian perfection leading up to their ultimate end: union with God (referred to as the body of Christ within God). According to Catholic theology, human nature has been weakened by original sin and is ever inclining toward what is evil; this end cannot be reached except at the price of overcoming many serious obstacles with God's grace.

The moral struggle involves overcoming spiritual obstacles, specifically the evil desires of the flesh, eyes, and pride of life, which are tests of original sin that try and challenge humanity (Trid., Sess. V, De peccato originali^{[incomplete short citation]}). The first duty, as described by the Apostle Paul in Ephesians 4:22, involves putting off "the old man". Next, he states that one must "put on the new man", which reflects the image of God. A Christian's obligation is to strive to be like Jesus, which requires divine grace. Baptism, whereby God adopts Christians as his children, bestows grace upon Christians through sanctification. Said sanctification must be perfected through supernatural virtues, the seven gifts of the Holy Spirit, and actual grace. Ascetics, at least in Catholicism, is, therefore, the systematic striving for Christian perfection.

Catholicism rejects the Protestant conception of essentially negative asceticism and that the correct notion of asceticism was discovered by the Reformers. There can be no doubt as to the Catholic position, clearly voiced by St. Thomas and St. Bonaventure who never tired of repeating that the ideal of asceticism upheld by them was the ideal of the Catholic past, of the Fathers, of Christ Himself, emphatically stating that bodily asceticism has not an absolute, but only a relative, value.^{[citation needed]} St. Thomas calls it a "means to an end", to be used with discretion. St. Bonaventure says that bodily austerities "prepare, foster and preserve perfection" ("Apolog. pauperum", V, c. viii). As proof he shows that to put an absolute value on bodily asceticism would lead to Manichæism. He also points to Christ, the ideal of Christian perfection, who was less austere in fasting than John the Baptist, and to the founders of religious orders, who prescribed fewer ascetic exercises for their communities than they themselves practised (cf. J. Zahn, "Vollkommenheitsideal" in "Moralprobleme", Freiburg, 1911, p. 126 sqq.). On the other hand, Catholics do not deny the importance of ascetic practices for acquiring Christian perfection. Considering the actual condition of human nature, they declare these necessary for the removal of obstacles and for the liberation of man's moral forces, thus claiming for asceticism a positive character. A like value is put upon those exercises which restrain and guide the powers of the soul. Consequently, Catholics actually fulfil and always have fulfilled what Harnack sets down as a demand of the Gospel and what he pretends to have looked for in vain among Catholics; for they do "wage battle against mammon, care, and selfishness, and practise that charity which loves to serve and to sacrifice itself" (Harnack, "Essence of Christianity"). The Catholic ideal, then, is by no means confined to the negative element of asceticism, but is of a positive nature.

The essence of Christian perfection is love. St. Thomas (Opusc. de perfectione christ., c. ii) calls that perfect which is conformable to its end (quod attingit ad finem ejus). As the end of man is God, what unites him, even on earth, most closely with God is love (1 Corinthians 6:17; 1 John 4:16). All the other virtues are subservient to love or its natural prerequisites, as faith and hope; Love seizes man's whole soul (intellect, will), sanctifies it and fuses new life into it. Love lives in all things and all things live in and through love. Love imparts to all things the right measure and directs them all to the last end. "Love is thus the principle of unity, no matter how diversified are the particular states, vocations and labours. There are many provinces, but they constitute one realm. The organs are many, but the organism is one" (Zahn, l. c., p. 146). Love is therefore rightly called "the bond of perfection" (Colossians 3:14) and the fulfilment of the law (Romans 13:8). That Christian perfection consists in love has ever been the teaching of Catholic ascetical writers. A few testimonies may suffice. Writing to the Corinthians, Clement of Rome says (1 Corinthians 49:1): "It was love that made all the elect perfect; without love nothing is acceptable to God" (en te agape ateleiothesan pantes oi eklektoi tou theou dicha agapes ouden euareston estin to theo; Funk, "Patr. apost.", p. 163). The Epistle of Barnabas insists that the way of light is "the love of him who created us" (agapeseis ton se poiesanta; Funk, l. c., p. 91), "a love of our neighbour that does not even spare our own life" (agapeseis ton plesion sou hyper ten psychen sou), and it affirms that perfection is nothing else than "love and joy over the good works which testify to justice" (agape euphrosyns kai agalliaseos ergon dikaiosynes martyria). St. Ignatius never wearies in his letters of proposing faith as the light and love as the way, love being the end and aim of faith ("Ad Ephes.", ix, xiv; "Ad Philad.", ix; "Ad Smyrn.", vi). According to the "Didache", love of God and of one's neighbour is the beginning of the "way of life" (c. i), and in the "Epistle to Diognetus" active love is called the fruit of belief in Christ. The "Pastor" of Hermas acknowledges the same ideal when he sets down "a life for God" (zoe to theo) as the sum-total of human existence. To these Apostolic Fathers may be added St. Ambrose (De fuga sæculi, c. iv, 17; c. vi, 35–36) and St. Augustine, who regards perfect justice as tantamount to perfect love. Both St. Thomas and St. Bonaventure speak the same language, and the ascetical writers of all subsequent centuries have faithfully followed in their authoritative footsteps (cf. Lutz, "Die kirchl. Lehre von den evang. Räten", Paderborn, 1907, pp. 26–99).

However, though perfection is essentially love, not any degree of love is sufficient to constitute moral perfection. The ethical perfection of the Christian consists in the perfection of love, which requires such a disposition "that we can act with speed and ease even though many obstacles obstruct our path" (Mutz, "Christl. Ascetik", 2nd ed., Paderborn, 1909). But this disposition of the soul supposes that the passions have been subdued; for it is the result of a laborious struggle, in which the moral virtues, steeled by love, force back and quell the evil inclinations and habits, supplanting them by good inclinations and habits. Only then has it really become "a man's second nature, as it were, to prove his love of God at certain times and under certain circumstances, to practise virtue and, as far as human nature may, to preserve his soul even from the slightest taints" (Mutz, l. c., p. 43). Owing to the weakness of human nature and the presence of the evil concupiscence (fomes peccati: Trid., Sess. VI, can. xxiii), a perfection that would exclude every defect cannot be attained in this life without a special privilege (cf. Proverbs 20:9; Ecclesiastes 7:21; James 3:2). Likewise, perfection on this side of the grave will never reach such a degree that further growth is impossible, as is clear from the mind of the Church and the nature of our present existence (status vioe); in other words, our perfection will always be relative. As St. Bernard says: "An unflagging zeal for advancing and a continual struggle for perfection is itself perfection" (Indefessus proficiendi studium et iugis conatus ad perfectionem, perfectio reputatur; "Ep. ccliv ad Abbatem Guarinum"). Since perfection consists in love, it is not the privilege of one particular state, but may be, and has as a fact been, attained in every state of life (cf. Christian and Religious Perfection). Consequently, it would be wrong to identify perfection with the so-called state of perfection and the observance of the evangelical counsels. As St. Thomas rightly observes, there are perfect men outside the religious orders and imperfect men within them (Summa theol., II-II, Q. clxxxiv, a. 4). True it is that the conditions for realizing the ideal of a Christian life are, generally speaking, more favourable in the religious state than in the secular avocations. But not all are called to the religious life, nor would all find in it their contentment. To sum up, the end is the same, the means are different. This sufficiently answers Harnack's objection (Essence of Christianity) that the Church considers the perfect imitation of Christ possible only for the monks, while she accounts the life of a Christian in the world as barely sufficient for the attainment of the last end.

The ideal, to which the Christian should conform and towards which he should strive with all his powers both natural and supernatural, is Jesus Christ. His whole life should be so penetrated by Christ that he becomes Christian in the full sense of the word ("until Christ be formed in you"; Galatians 4:19). That Christ is the supreme model and pattern of the Christian life follows from Scripture, e. g. from John, xiii, 15, and I Peter, ii, 21, where imitation of Christ is directly recommended, and from John, viii, 12, where Christ is called "the light of the world". Cf. also Rom., viii, 29, Gal., ii, 20, Phil., iii, 8, and Heb., i, 3, where the Apostle extols the excellent knowledge of Jesus Christ, for whom he has suffered the loss of all things, counting them but as dung, that he may gain Christ. Of the numerous testimonies of the Fathers we only quote St. Augustine: "Finis ergo noster perfectio nostra esse debet; perfectio nostra Christus" (P. L., XXXVI, 628; cf. also "In Psalm.", 26, 2, in P. L., XXXVI, 662). In Christ there is no shadow, nothing one-sided. His Divinity guarantees the purity of the model; His humanity, by which He became similar to us, makes the model attractive. But this picture of Christ, unmarred by addition or omission, is to be found only in the Catholic Church and, owing to her indefectibility, will always continue there in its ideal state. For the same reason, the Church alone can give us the guarantee that the ideal of the Christian life will always remain pure and unadulterated, and will not be identified with one particular state or with a subordinate virtue (cf. Zahn, l. c., p. 124). An unprejudiced examination proves that the ideal of Catholic life has been preserved in all its purity through the centuries and that the Church has never failed to correct the false touches with which individuals might have sought to disfigure its unstained beauty. The individual features and the fresh colours for outlining the living picture of Christ are derived from the sources of Revelation and the doctrinal decisions of the Church. These tell us about the internal sanctity of Christ (John 1:14; Colossians 2:9; Hebrews 1:9; etc.). His life overflowing with grace, of whose fulness we have all received (John 1:16), His life of prayer (Mark 1:21, 35; 3:1; Luke 5:16; 6:12; 9:18; etc.), His devotion to His heavenly Father (Matthew 11:26; John 4:34; 5:30; 8:26, 29), His intercourse with men (Matthew 9:10; cf. 1 Corinthians 9:22), His spirit of unselfishness and sacrifice, His patience and meekness, and, finally, His asceticism as revealed in his fastings (Matthew 4:2; 6:18).