Heracleon was a Gnostic who flourished about AD 175, probably in the south of Italy. He is the author of the earliest known commentary on a book that would eventually be included in the Christian New Testament with his commentary on the Gospel of John, although only fragmentary quotes survive. He is described by Clement of Alexandria (Strom. 4.9) as the most esteemed (δοκιμώτατος) of the school of Valentinus; and, according to Origen (Comm. in S. Joann. t. ii. § 8, Opp. t. iv. p. 66), said to have been in personal contact (γνώριμος) with Valentinus himself. He is barely mentioned by Irenaeus (2.4.1) and by Tertullian (adv. Valent. 4). The common source of Philaster and Pseudo-Tertullian (i.e. probably the earlier treatise of Hippolytus) contained an article on Heracleon between those on Ptolemaeus and Secundus, and on Marcus and Colarbasus.

In his system he appears to have regarded the divine nature as a vast abyss in whose Pleroma were Aeons of different orders and/or degrees, emanations from the source of being. Midway between the supreme God and the material world was the Demiurge, who created the latter, and under whose jurisdiction the lower, animal soul of man proceeded after death, while his higher, celestial soul returned to the Pleroma whence at first it issued.

He seems to have received the ordinary Christian scriptures; and Origen, who treats him as a notable exegete, has preserved fragments of Heracleon's commentary on the Gospel of John, while Clement of Alexandria quotes from him what appears to be a passage from a commentary on the Gospel of Luke. These writings have intensely mystical and allegorical interpretations of the text.

Neander and Cave have suggested Alexandria as the place where Heracleon taught; but Clement's language suggests some distance either of time or of place; for he would scarcely have thought it necessary to explain that Heracleon was the most in repute of the Valentinians if he were at the time the head of a rival school in the same city. Hippolytus makes Heracleon one of the Italian school of Valentinians; but the silence of all the authorities makes it unlikely that he taught at Rome. It seems, therefore, most likely that he taught in one of the cities of S. Italy; or Praedestinatus may be right in making Sicily the scene of his inventions about Heracleon.

The date of Heracleon is of interest on account of his use of St. John's Gospel, which clearly had attained high authority when he wrote. The mere fact, however, that a book was held in equal honour by the Valentinians and the orthodox seems to prove that it must have attained its position before the separation of the Valentinians from the church; and, if so, it is of less importance to determine the exact date of Heracleon. The decade 170–180 may probably be fixed for the centre of his activity. This would not be inconsistent with his having been personally instructed by Valentinus, who continued to teach as late as 160, and would allow time for Heracleon to have gained celebrity before Clement wrote, one of whose references to Heracleon is in what was probably one of his earliest works. He had evidently long passed from the scene when Origen wrote.

The chief interest that now attaches to Heracleon is that he is the earliest commentator on the N.T. of whom we have knowledge. Origen, in the still extant portion of his commentary on St. John, quotes Heracleon nearly 50 times, usually controverting, occasionally accepting his expositions. We thus recover large sections of Heracleon's commentary on cc. i. ii. iv. and viii. of St. John. There is reason to think that he wrote commentaries on St. Luke also. Clement of Alexandria (Strom. iv. 9) expressly quotes from Heracleon's exposition of Luke 12:8; and another reference (25 Eclog. ex Script. Proph. p. 995) is in connexion with Luke 3:16–17, and so probably from an exposition of these verses.

The first passage quoted by Clement bears on an accusation brought against some of the Gnostic sects, that they taught that it was no sin to avoid martyrdom by denying the faith. No exception can be taken to what Heracleon says on this subject.

Men mistake in thinking that the only confession is that made with the voice before the magistrates; there is another confession made in the life and conversation, by faith and works corresponding to the faith. The first confession may be made by a hypocrite: and it is one not required of all; there are many who have never been called on to make it, as for instance Matthew, Philip, Thomas, Levi [Lebbaeus]; the other confession must be made by all. He who has first confessed in his disposition of heart will confess with the voice also when need shall arise and reason require. Well did Christ use concerning confession the phrase 'in Me' (ἐὰν ὁμολογήσῃ ἐν ἐμοί), concerning denial the phrase 'Me.' A man may confess 'Him' with the voice who really denies Him, if he does not confess Him also in action; but those only confess 'in Him' who live in the confession and in corresponding actions. Nay, it is He Whom they embrace and Who dwells in them Who makes confession 'in them'; for 'He cannot deny Himself.' But concerning denial, He did not say whosoever shall deny 'in Me,' but whosoever shall deny 'Me'; for no one that is 'in Him' can deny Him. And the words 'before men' do not mean before unbelievers only, but before Christians and unbelievers alike; before the one by their life and conversation, before the others in words.