Because I had a personal experience of God, and because I believe based on the arguments of evidence and logic.
I was once a very ardent atheist. I am very much unrecognizable from the man I was 12 years ago. I also walked a very self destructive path in my life. I’m not saying the two are related one to another, but that they were in my case.
It took someone patient enough to answer a lot of hard questions I had used for a long time to justify my disbelief, and humble enough to admit they did not know, and dedicated enough to find answers to bring me, before my heart started to soften.
The effect was very immediate on me. The way I thought and acted, and the way I treated those around me radically began to change. Hurts and issues I had carried my entire life began to wash away. It was a life changing thing to say the least. I reconciled relationships that had been broken for a decade.
I try really hard to not push what I’ve experienced or believe on other people out of respect, but also because I know that would certainly not have helped me. But I would be a coward to deny Christ publicly, so I opted for a simple public profession of my faith.