When it comes to authorship of the Bible, of course men were involved. Christians would be the first to point this out. For example, Paul wrote letters to early churches that are included in the Scriptures (2 Peter 3:1516). David wrote many of the Psalms. Moses wrote the Pentateuch, or the Torah (the first five books of the Bible). In fact, it is estimated that over 40 different human authors were involved.2 So, this is not the issue.
The issue is this: did God have any involvement or not? Did God inspire the authors of the Scriptures?3 When someone claims that the Bible was written by men and not God, this is an absolute statement that reveals something extraordinary.
It reveals that the person saying this is claiming to be transcendent! When one claims that God was not inspiring the human authors of the Bible, that person is claiming to be omniscient, omnipresent, and omnipotent!
Omniscient: they are claiming to be an all-knowing authority on the subject of God’s inspiration, to refute God’s claim that Scripture was inspired by Him (2 Timothy 3:16).
Omnipresent: they are claiming that they were present, both spiritually and physically, to observe that God had no part in aiding any of the biblical authors.
Omnipotent: they are claiming that if God had tried to help the biblical authors, then they had the power to stop such an action.
So, the person making the claim that the Bible was written by men is claiming to be God; but these three attributes belong solely to God. This is a religious issue of humanism versus Christianity. The person is claiming (perhaps inadvertently) that they are the ultimate authority over God and are trying to convince you that God is subservient to them. This needs to be addressed in responding to them.