Error, indeed, is never set forth in its naked deformity, lest, being thus exposed, it should at once be detected. But it is craftily decked out in an attractive dress, so as, by its outward form, to make it appear to the inexperienced (ridiculous as the expression may seem) more true than the truth itself.

In his preface to Book I, Irenaeus asserts that the gnostics deceive people by making their heresies look attractive and true. He makes the point that to be successful, lies must masquerade as truths. He goes on to say that his goal is especially to refute the "absurd" and "inconsistent" teachings of "the disciples of Ptolemaeus, whose school may be described as a bud from that of Valentinus." Irenaeus also asserts his humility as a one of "moderate ability," which, of course, is not true: the book shows the sophisticated level of his theology.

Christ Jesus, the Son of God; who, because of His surpassing love towards His creation, condescended to be born of the virgin, He Himself uniting man through Himself to God, and having suffered under Pontius Pilate, and rising again, and having been received up in splendour, shall come in glory, the Saviour of those who are saved, and the Judge of those who are judged, and sending into eternal fire those who transform the truth, and despise His Father and His advent.

More than one hundred years before the Emperor Constantine brought leading theologians across the Empire together to hash out the Nicene Creed, a foundational statement of Christian faith, Irenaeus articulated some of its main tenets in the quote above. These are important in the argument against gnosticism that Ireneaus is building. The gnostics claimed for example, that Jesus was a wholly spiritual being who did suffer on the cross (his physical body being essentially an illusion). In this passage, Irenaeus asserts unequivocally that Jesus was born as a human and "suffered under Pontius Pilate."

For who else is there who can reign uninterruptedly over the house of Jacob for ever, except Jesus Christ our Lord, the Son of the Most High God, who promised by the law and the prophets that He would make His salvation visible to all flesh; so that He would become the Son of man for this purpose, that man also might become the son of God?

The last part of this statement is one of the most famous sayings of Irenaeus: in it Irenaeus asserts that Jesus "would become the Son of man . . . that man also might become the son of God." In other words, Jesus brings humankind into intimate relationship with God. However, the first part of the statement is also important in showing that the Old Testament (often now called the Hebrew Bible) foretold the coming of Christ. By closely linking the two testaments, Irenaeus corrected those who said the God of the Old Testament was different from the God of the New.