Most errors of thought involve not so much a total rejection of truth, but assenting to one extreme end of a balanced truth while rejecting the other. (The history of heresy is often a battle between two extreme views each trying to counter the other when the truth is a mid-point between them.) Such is the case with faith and reason.
The human soul has two basic powers: intellect and will. Reasoning is the activity of the intellect, and its product is knowledge. Faith is the result of a choice made by the will to trust in God’s revelation. So faith is not knowledge – even though what is believed on faith is more trustworthy than what we believe by reason because of the trustworthiness of its source (God, who is all truth). The balance between faith and reason comes when we consider that different truths are revealed in different ways.
Some things (like the Trinity) must be revealed for us, because we cannot know them naturally (e.g., Matthew 16:9). We call this “supernatural revelation” because these truths can only be revealed directly by God (who is beyond nature). Some people think only these kinds of truths are trustworthy – this is a view called Fideism.
Other things are knowable apart from faith (like math). We call this “natural revelation” because these truths are revealed in nature—even many things about God (see Romans 1:19-20). Some people think only these kinds of truths can be known, though – and this is a view called Rationalism.
Finally, some truths overlap in the way they can be discovered (like the location of Samaria — e.g., John 4:1-6), so there must be a third view in between the two above extremes that best describes reality and how we come to know it. What the Church has long recognized is that both sets of truths are part of God’s revelation, and so they cannot truly conflict with one another. Both, therefore, should be trusted.
When it comes to religion, if we go too far in one direction, we get an unreasonable Fideism which rejects God’s natural revelation. If we go too far in the other direction, we arrive at an unfaithful Rationalism which rejects God’s supernatural revelation. Neither is acceptable for a Christian – rather, we hold to total, universal truths as revealed by God in whichever way he chooses to reveal them. As St. John Paul II said in his brilliant encyclical Fides et Ratio (“Faith and Reason”):
“Faith and reason are like two wings on which the human spirit rises to the contemplation of truth.”