I wrote this when I was more of an evidentialist ...
The question of whether or not God exists is extremely important. It has more ramifications that flow from the answer than any other question. This is a large question and I will not be able to go into much details as I would like; for example, I will not be able to talk much about the internal witness of conscience or the way the Holy Spirit communicates with a person's heart. Not that I think my opponent would find those argument compelling, naturally, but they are still very important.
I want to immediately speak of the fact of dependency. Everything that exists that we know of began to exist at a certain time. Since there was a time when that thing was not, this means its existence is not actually necessary because things existed already before that thing. For example, I didn't have to exist, the Universe could function naturally without me. In fact, my coming into existence was dependent upon my mom and dad and likewise they with their parents and all the way back. So, if everything is dependent upon its existence on something that came before it, how did anything get here at all? Somewhere the mere fact of existence demands an utterly independent being, whose existence is not contingent upon anything else. We call this being God.
Another way to think of this is that nothing is actually a pure "being". Technically, we are humans becoming, not human beings. What this means is we are never the same – everything about us is always changing in every way. Another way to phrase this fact is that we exist but our essence is not existence. Somewhere along the line, we need a being who is not becoming but rather pure being. This being's essence would have to be existence itself – hence, the Covenant name for the God of the Bible – "I AM".