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)