Almighty God, who didst uphold thy servant Irenaeus with strength to maintain the truth against every blast of vain doctrine: Keep us, we beseech thee, steadfast in thy true religion, that in constancy and peace we may walk in the way that leadeth to eternal life; through Jesus Christ our Lord, who liveth and reigneth with thee and the Holy Spirit, one God, for ever and ever.
To celebrate, read books 4-6 of Against Heresies by St. Irenaeus. Good in any century.