Take a few minutes to find out what Biblical Christianity teaches on that good question in the links of my signature. A man in the N.T. asked a very similar question.
A non believer may know what Christianity teaches, may have read the bible, but is not convinced....what then?
Just like a lot of things in life, it sometimes takes an open mind and investigation. I like the gospel of John aka St. John as a place to spend the time to anyone searching. It claims to be the one unique book written to convince open-minded unbelievers.
There are examples of unbelievers who actually ask God for insight and He answers those seeking truth of Him.
The quote came to mind when you asked your other question was from Acts 16.
"And brought them out, and said, Sirs, what must I do to be saved? And they said, Believe on the Lord Jesus Christ, and thou shalt be saved, and thy house." Meaning that his household will be saved if they also choose to believe on Jesus Christ.
It's easy to accuse those who disagree with one's beliefs of a closed mind. It may be the believer who is unwilling to consider other possibilities.
Nobody argues over matters that have been established through evidence.
No reasonable person would doubt the existence of a God given sufficient evidence.
Perhaps I should have put it a different way. A person can be open minded about a lot of things, including the religious/ metaphysical. I meant that concerning those open to the possibility of the existence of a God described by the Bible. PS:...and the reconciliation of God and men through His Son.
