Truth cannot contradict truth. This is nothing more than the law of non-contradiction. Consequently there can be no genuine conflict between faith and reason. Pope John Paul II makes this point in Fides et Ratio §16:
What is distinctive in the biblical text is the conviction that there is a profound and indissoluble unity between the knowledge of reason and the knowledge of faith. The world and all that happens within it, including history and the fate of peoples, are realities to be observed, analyzed and assessed with all the resources of reason, but without faith ever being foreign to the process. Faith intervenes not to abolish reason’s autonomy nor to reduce its scope for action, but solely to bring the human being to understand that in these events it is the God of Israel who acts. Thus the world and the events of history cannot be understood in depth without professing faith in the God who is at work in them.
Consequently when we are faced with an apparent conflict between the two we must realize that it is only apparent, and seek a resolution as best we can. Furthermore, because reasoning is difficult and because revelation is certain by reason of the fact that God can neither err nor lie, we need to keep in mind that—barring a misunderstanding on our part concerning the content of revelation—the more likely case is that we (or someone) has erred in the use of reason. See what St Thomas has to say about this here:
Even as regards those truths about God which human reason could have discovered, it was necessary that man should be taught by a divine revelation; because the truth about God such as reason could discover, would only be known by a few, and that after a long time, and with the admixture of many errors. [ST I q1 a1]
Consequently we ought not to be afraid to use our heads, and we ought not to be afraid to believe what the God has revealed and has caused to be preserved and proclaimed by the Church. But if this is so, then there is no good reason for anyone to object to belief in God on rationalistic grounds:
If human beings with their intelligence fail to recognize God as Creator of all, it is not because they lack the means to do so, but because their free will and their sinfulness place an impediment in the way. [FR §19]
John Paul picks up this theme again in §34:
The unity of truth is a fundamental premise of human reasoning, as the principle of non-contradiction makes clear. Revelation renders this unity certain, showing that the God of creation is also the God of salvation history. It is the one and the same God who establishes and guarantees the intelligibility and reasonableness of the natural order of things upon which scientists confidently depend, and who reveals himself as the Father of our Lord Jesus Christ. This unity of truth, natural and revealed, is embodied in a living and personal way in Christ. [Emphasis added]
Truth is one. The world is intelligible because God made it and us.
Aquinas, Etc. 
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