"there is no sure-fire way to prove God's existence (or lack thereof)"
That's a comment by some atheist this morning. I think a more accurate statement would be,
There is no sure-fire way to prove God's existence to another person.
God's existence HAS been proven to me by God Himself. However, helping someone else acknowledge His reality is not possible. That's the work of the Holy Spirit and He moves where He will as He desires.