si peccasse negamus fallimur et nulla est in nobis veritas
if we refuse to make a mistake, we are deceived, and there's no truth in us
Notes
From Christopher Marlowe's The Tragical History of Doctor Faustus, where the phrase is translated "if we say that we have no sin, we deceive ourselves, and there's no truth in us". (cf. 1 John 1:8 in the New Testament)