Once upon a time, Tertullian is supposed to have said "credo quia absurdum est", which is Latin for "I believe because it is absurd." But as intimated, this is an atheist fairy tale, for Tertullian never said it, at least not the Tertullian of history who is an altoghether different person from the Tertullian of atheist-faith.
Of course if it magically turned out that Tertullian did say such a thing, it would hardly be damaging to the cause of Christ since other views have been held among believers, and much more widely, such as the Thomistic view - intelligo ut creedam ("I understand in order to believe"), and also the Augustinian view - credo ut intelligam ("I believe in order to understand").
According to the Thomistic view, reason precedes understanding and is made the foundation for faith; and according to the Augustinian view, faith is seen as the precondition of reason, that without which there can be no true understanding (because reason would then have no pou sto or fixed reference point for predication).
It is quite apparent from the whole scope of his writings that Tertullian was more of a proto-Augustinian than anything else on this matter, and one of the better single examples of this from his writings is the following:
"For philosophy is the material of the world’s wisdom, the rash interpreter of the nature and dispensation of God. Indeed heresies are themselves instigated by philosophy… What indeed has Athens to do with Jerusalem? What has the Academy to do with the Church? What have heretics to do with Christians? Our instruction comes from the porch of Solomon, who had himself taught that the Lord should be sought in simplicity of heart. Away with all attempts to produce a Stoic, Platonic, and dialectic Christianity! We want no curious disputation after possessing Christ Jesus, no inquisition after receiving the gospel! When we believe, we desire no further belief. For this is our first article of faith, that there is nothing which we ought to believe besides." (The Prescription Against Heretics, VII.)
The above quote does reveal that Tertullian posited a stark antithesis between the world's wisdom, starting from its own autonomous speculations or reasoning about things (represented by Athens), and the wisdom that is from above (represented by Jerusalem), the reason that begins with God's revelation and, in simplicity of heart, goes on to produce an orthodoxy without any admixture of foreign elements. Nevertheless, it does not show a rejection of reason or advocate the notion that faith is antithetical to reason, unless that reason takes unbridled speculation, rather than God's revelation, as its starting point. Indeed, it was the same Tertullian who said the following about reason, rooting reason in God and seeing the world as open to rational investigation for this very reason:
"For reason is a property of God's, since there is nothing which God, the creator of all things, has not foreseen, arranged and determined by reason; moreover, there is nothing He does not wish to be investigated and understood by reason." (On Repentance, Ch. 1, vs. 2)
In any case, Tertullian never said that he was a Christian because Christianity is absurd. The closest one can get to a comment like this is the following from his De Carni Christi ("On the Flesh of Christ") written against the heretic Marcion:
"The Son of God was crucified: I am not ashamed--because it is shameful.
All sorts of considerations factor into a correct understanding of what is seen here, but the most relevant thing, beyond the fact that it isn't the same as what atheists and others have quoted Tertullian as saying, is the following: it is just because the central claims of the Christian Gospel are, from the perspective of the world's wisdom, shameful, silly, and impossible, that they are, from the perspective of that wisdom that is from above, altogether believable. In other words, unbelieving and believing wisdom have two different starting points and therefore end up evaluating the same thing very differently.
[As Paul said in 1 Corinthians 1, unbelieving wisdom, starting from a rejection of God, or at least, leaving God out of account, and reasoning according to its own crooked standards, will always deem God's revealed truth as weak and foolish, while believers will see it as the power of God and the wisdom of God. Given the antithesis between believers and unbelievers, given that unbelievers begin with themselves as autonomous, while believers begin with God as absolute, given that unbelieving wisdom proceeds according to its own darkened understanding and believing wisdom is lit up by the revelation of God, they will always have radically different evaluations of such things.]
This understanding is bolstered by the fact that the context of Tertullian's remarks are in fact in reference to 1 Corinthians, where it is written:
"For Christ did not send me to baptize, but to preach the gospel, not in cleverness of speech, so that the cross of Christ would not be made void. For the word of the cross is foolishness to those who are perishing, but to us who are being saved it is the power of God. For it is written, 'I will destroy the wisdom of the wise, and the cleverness of the clever I will set aside.' Where is the wise man? Where is the scribe? Where is the debater of this age? Has not God made foolish the wisdom of the world? For since in the wisdom of God the world through its wisdom did not come to know God, God was well-pleased through the foolishness of the message preached to save those who believe. For indeed Jews ask for signs and Greeks search for wisdom; but we preach Christ crucified, to Jews a stumbling block and to Gentiles foolishness, but to those who are the called, both Jews and Greeks, Christ the power of God and the wisdom of God. Because the foolishness of God is wiser than men, and the weakness of God is stronger than men."
A somewhat different interpretation of this passage in De Carni Christi, one that is not contrary but complementary to the overarching (proto-)Augustinian approach of Tertullian that was briefly sketched above, has also been suggested and is a widely held interpretation. Lindberg and Numbers explain it this way:
"...Tertullian was simply making use of a standard Aristotelian argumentative form, maintaining that the more improbable an event, the less likely is anybody to believe, without compelling evidence, that it has occurred; therefore, the very improbability of an alleged event, such as Christ's resurrection, is evidence in its favor." (Lindberg and Numbers, God & Nature: Historical Essays on the Encounter Between Christianity and Science, p. 26)
In a footnote (#34) to this, the authors go on to explain:
"That is, resurrection of the dead is so improbable an event that the apostles would not have believed in the resurrection of Christ if they had not been faced with incontrovertible evidence that indeed, on this occasion, the improbable occurred. This fact makes the resurrection of Christ more probable than some other event; the occurrence of which might have been accepted merely on the basis of general plausibility".