Well, I based my atheism firmly on science and philosophical arguments against the existence of God. I realized that science demands a creator, that there must be a metaphysical and something beyond the laws of science. The universe is too orderly to have been chaotically and randomly spawned from a nothingness. What we live in is a creation, and like all creations it needs a creator. This helped me understand and believe that a higher power exists. More specifically I believed in Christianity because the Bible is proven true through archeology and other areas of science and history (which can be argued, but looking at the facts presented I do strongly believe that science and history support the Bible). Also, the Holy Spirit led my heart to Christ through a series of events. I came to the belief that there is a God, and that because there is a God there must be a truth about God, and I believe that truth is Christianity. If you want I can explain more through PMs (I don't want to change the focus of the thread or hijack it).
Also, I believe in absolutes mainly because of my faith, but also because philosophically it makes sense. There are some things that are absolutes, we can have opinions and perceptions, but there is only 1 ultimate truth that we have an opinion about and perceive. I don't think truth is relative nor is truth dictated by humanity, I believe truth was set down by God. One reason I don't think truth is relative though is because there are some paradoxes to that statement. If truth is relative then all truths should be equal. If truth is based on what we believe it to be, then one person's truth is equal to another person's truth. They both base their truth on experience, perceptions, and things they already know and believe. If all truth is equal though, then what happens when one person's truth contradicts another? What if someone believes when we die we are re-incarnated while another believes in a personal afterlife? Who is right? They can't both be right, because for one person to be right the other must be wrong. Reality doesn't operate based on how we want it to, reality is unchanging regardless of what our opinions of truth are. One can believe that the sky is green, and it may be his truth, but reality will always remain absolute regardless of what this person believes. We can hold an opinion about something, but our opinion doesn't alter or shape what reality and truth really are. In essence, they are absolutes independent of our opinions or beliefs.