What is Faith?
There's natural faith and supernatural faith. Natural faith is believing that your car will start or that your team will win. But I think you are asking about supernatural faith. Supernatural faith is prompted by God's power working on my soul to make me believe in God's existence and everything He has revealed for our belief. Supernatural faith is a gift we can't initially earn, but we can lose it if we fail to co-operate with God's grace. Faith without works is dead (James 2: 26; CCC 1815). Faith is not opposed to reason; it is beyond reason because faith's object is God Who is infinitely beyond our finite reason.
* Please note that this text should be read in the context of the whole work and in recognition of the appropriate paragraphs of the Catechism of the Catholic Church highlighted in the index.