Today is the feast day of the ancient Christian bishop, apologist and Church Father Irenaeus of Lyon.

For early followers of Jesus, this figure was formative: Without his work, the Church wouldn't be what it is today. Raised as a Christian, Irenaeus soon became a priest, and later a bishop, at a time when the Church still faced intense oppression.

His most famous work is Adversus Haereses, in English 'Against Heresies', where he made his case for Christian Orthodoxy and combatted the teachings of the Gnostic tradition. The Gnostics were a diverse, mysterious group, but amongst their beliefs was a belief in a Christ who only appeared to be human, wasn't truly crucified, and came to grant believers secret knowledge (gnosis) and enable their escape from a corrupt material existence into a purely spiritual realm.

They had their own Gospels too, and Irenaeus made great efforts to show how they erred. He affirmed the four Gospels that Christians use today as canon, for the first time.

He died on this day in AD 203, but today is celebrated as a saint in the Catholic and Eastern Orthodox churches. Here are nine quotes to remember him by.

1. He who was the Son of God became the Son of man, that man ... might become the son of God.

2. The glory of God is a human being fully alive; and to be alive consists in beholding God.

3. The business of the Christian is nothing else but to be ever preparing for death.

4. Jesus Christ, in His infinite love, has become what we are, in order that He may make us entirely what He is.

5. One should not seek among others the truth that can be easily gotten from the Church. For in her, as in a rich treasury, the apostles have placed all that pertains to truth, so that everyone can drink this beverage of life. She is the door of life.

6. 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 more true than truth itself.

7. When we stand in the light it is not we who illumine the light and cause it to shine but we are illuminated and made shining by the light... God grants his blessings on those who serve him because they are serving him and on those who follow him because they are following him, but he receives no blessing from them because he is perfect and without need.

8. As long as any one has the means of doing good to his neighbours, and does not do so, he shall be reckoned a stranger to the love of the Lord.

9. He [Jesus] fought and conquered. On the one hand, he was man who struggled for his fathers and through his obedience cancelled their disobedience. On the other hand, he bound the strong one and freed the weak and bestowed salvation on his handiwork by abolishing sin. For he is our compassionate and merciful Lord who loves mankind ... Had not man conquered man's adversary, the enemy would not have been conquered justly. Again, had it not been God who bestowed salvation we would not possess it securely.