In my opinion, it is never a waste of time or brain power when discussing matters of eternal value.
Besides, the onus is on YOU to prove that God doesn't exist, is it not?
How do we know that God exists? Are there good reasons for believing in God, or must His existence be accepted on blind faith?

The Bible does not support the idea of “blind faith,” or faith as a “leap into the dark.” Biblical faith is a faith that sees what those who have no faith fail to see; it is a leap into the light, not into the dark. For Christians, God is not a theoretical abstraction, but Someone who loves us and who has made Himself known to us.

Thus, the bottom line is that we know God exists because He has revealed Himself to us. He has done this in many ways — through creation, through our moral consciousness, through miraculous experiences, and in many other ways. Philosophers have designed arguments that spell out the logic of these testimonies to God’s existence, but these arguments simply formalize what should be intuitively obvious to anyone who looks around and within themselves.

In short, there are many arguments for the existence of God, most of them valid and quite powerful. However, although these many arguments for the existence of God are substantial, because we are sinners and cut off from immediate communion with God we need something more than good arguments. What we need is for God to reveal Himself to the world. This is exactly what He has done. First through prophets in the Bible, and then fully and concretely in Jesus Christ, God has spoken to the world, told us about Himself, shown us what He is like, and called us to be reconciled to Him through the death of His Son, giving proof to the world by raising Jesus from the dead and through Him fulfilling numerous Old Testament prophecies. And that is how we know that God exists.