FAUSTUS: Si peccasse negamus, fallimur, et nulla est in nobis veritas;
“If we say that we have no sin,
We deceive ourselves, and there’s no truth in us.”
Why, then, belike we must sin,
And so consequently die.
Ay, we must die an everlasting death.

Christopher "Kit" Marlowe (1564–1593) English dramatist and poet

The Tragicall History of the Life and Death of Doctor Faustus, Act 1, sc. 1 (sc. 1), l. 70ff (1594; 1604 “A” text)
    (Source)

The quote is from the Bible, 1 John 1:8; Faustus ignores verse 9 which speaks of forgiveness.

The same words are used in the "B" text (w. 1594; pub. 1616), l. 68ff.