Natus est Dei Filius; non pudet, quia pudendum est: et mortuus est Dei Filius; prorsus credibile est, quia ineptum est: et sepultus resurrexit; certum est, quia impossibile.
"The Son of God was born: there is no shame, because it is shameful. And the Son of God died: it is wholly credible, because it is ridiculous. And, buried, He rose again: it is certain, because impossible."
The phrase is sometimes associated incorrectly with the doctrine of fideism, that is, "a system of philosophy or an attitude of mind, which, denying the power of unaided human reason to reach certitude, affirms that the fundamental act of human knowledge consists in an act of faith, and the supreme criterion of certitude is authority." (Catholic Encyclopedia). Fideism as a school of thought was rejected by Church in the Middle Ages.
Tertullian's quotation actually implies a rejection of Fideism, in that he asserts that the Apostles, being reasonable people, would not have believed in something as incredible as the resurrection of Jesus Christ had they not seen it firsthand.
However, the line "What I believe in cannot be proven" can be taken neither as a statement of Fideism nor a statement against it, unless the nature of the unprovable be evaluated in some way. Tertullian thought that reasonable people will not accept contradictory assertions and will dismiss them as ridiculous or impossible unless they themselves have legitimate certainty concerning them.
The Fideist, on the other hand, would believe in something whether it were unprovable or not, because the Fideist -- mistrusting any reason -- would in fact have no means by which to prove or disprove such an assertion, and so would necessarily have recourse solely to authority. This stance finds nothing in common with the Catholic theological and intellectual tradition of Tertullian's time, of the time of St. Thomas, or of our own day. Of course, since the Catholic Church claims that this intellectual tradition has undergone no changes since Tertullian's time, and the Church rejected Fideism, the cited passage must mean that `the Apostles, being reasonable people, would not have believed in something as incredible as the resurrection of Jesus Christ had they not seen it firsthand.'
In Civilization and Its Discontents, Freud reverts to the phrase while questioning the commandment 'Thou shalt love thy neighbor as thyself'.
It has also been used, though often in different interpretations, by some existentialists.