In the Catholic Church, an altar is a table or structure on which Mass is celebrated. Relics of martyrs or other saints are commonly set into the altar. Typically centrally located in the sanctuary, the altar is to be the focus of attention in the church. At the beginning of the Roman Rite of Mass, the priest first of all reverences the altar with a kiss and only after that goes to the chair at which he presides over the Introductory Rites and the Liturgy of the Word. Except in a Solemn Mass, a priest celebrating the Tridentine Mass remains at the altar the whole time after saying the prayers at the foot of the altar.

The rite of dedication of a church and of the altar points out that the celebration of the Eucharist is "the principal and the most ancient part of the whole rite, because the celebration of the eucharist is in the closest harmony with the rite of the dedication of a church", and "the eucharist, which sanctifies the hearts of those who receive it, in a sense consecrates the altar and the place of celebration, as the ancient Fathers of the Church often assert: 'This altar should be an object of awe: by nature it is stone, but it is made holy when it receives the body of Christ.'"

In Greek and some other languages used in the Byzantine Rite, the same word (βωμός) is used for an altar (in general) and for the area surrounding it; that is to say, the entire sanctuary. To refer unambiguously to the altar itself the terms "Holy Table" (Ἁγία Τράπεζα) or "Throne" (Church Slavonic: Prestólcode: chu promoted to code: cu ) are used.

Augustin Joseph Schulte says that Pope Sixtus II (257–259) was the first to prescribe that Mass should be celebrated on an altar, and that there are accounts according to which Lucian of Antioch celebrated Mass on his breast whilst in prison (312), and Theodore, Bishop of Tyre on the hands of his deacons.

Early Christians faced east at prayer, a practice witnessed to by Clement of Alexandria (c. 150 – c. 215), Tertullian (c. 160 – c. 220), and Origen (c. 185 – 253). Churches were generally built with an east–west axis. In the earliest churches in Rome the altar stood at the west end and the priest stood at the western side of the altar facing east and facing the people and the doors of the church. Examples are the Constantinian St. Peter's Basilica and the original Basilica of Saint Paul Outside the Walls. In the East, early churches had the altar at the east end and the priest, facing east, stood at the western side of the altar, with his back to the people and the doors. This later became the common practice also in western Europe. It was adopted in Rome in the 8th or 9th century.

The churches that Christians built after the legalization of their religion in the Roman Empire were not modelled on pagan temples, which were not intended to accommodate large numbers of people. The model used was that of the public basilicas that served for meetings such as sessions of law courts. These were generally spacious, and the interior was divided by two or four rows of pillars, forming a central nave and side aisles. At the end was a raised platform, often situated in an apse, with seats for the magistrates. In basilica-style Christian churches the apse was reserved for the bishop and his clergy; the faithful occupied the centre and the side aisles, and between the clergy and people stood the altar.

The earliest altars for celebrating the Christian Eucharist were of wood and identical in form with ordinary house tables, as was doubtless used at the Last Supper. The only such ancient wooden table still preserved is in the Lateran Basilica, and fragments of another are preserved in the Santa Pudenziana church in Rome. A tradition that lacks convincing evidence says that Peter celebrated the Eucharist on both. Optatus of Mileve reproves the Donatists for breaking up and using for firewood the altars of the Catholic churches, and Augustine of Hippo reports that Bishop Maximianus was beaten with the wood of the altar under which he had taken refuge.

Helena (c.  250 – c. 330) gave golden altars ornamented with precious stones to the original Church of the Holy Sepulchre. Pulcheria (398 or 399 – 453), sister of Theodosius II, presented an altar of gold to the Basilica of Constantinople. Popes Sixtus III (432–440) and Hilary (461–468) presented several altars of silver to the churches of Rome.