“For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness.”
An interesting question to ask skeptics, when you are defending the faith to them, is this: “How much do you really know?” You can ask further: “Is there anything that you absolutely know for sure?”
If they answer “Yes”, then you are going to ask them how they know it for sure, and show them that there is no firm foundation for their knowledge. Even something as obvious as two times three equals six requires there to be an assumption that this will always be true. How do we know?
Suppose you had one percent of all the knowledge in the universe. Could there possibly be anything in the other 99% of all knowledge that contradicts the 1% that you have? Most people would say that is true. But if that is true, then you don’t really know that 1% of knowledge for sure after all! It is something that is probably true, not certainly true. In this exercise, we are not interested in probable truth.
The only realistic way to know something for sure is if we know someone else who knows everything there is to know, who is completely trustworthy, and who has revealed some of that knowledge to us. And that is precisely our position as Christians. God knows everything because He is omnipotent, omniscient and omnipresent. And He has chosen to reveal truth to us through the Bible. So we can be 100% sure of that knowledge because we are 100% sure of Him.