"How can the Bible be infallible when it was written by 40 different men?"
"It's as relevant today as the day it was written."
That is one reason we know that it is infallible! The Bible was written in a timespan of more than 1,600 years and by different men from different professions and different times, and yet they all say the same thing.
The only way to make sense of this is that there's only one author who used all these men to write the book. We don't deny the fact that men have penned the different books; this is proof that it is a God-dictated book. A book can become irrelevant within six months. But look at the Bible.
It's as relevant today as the day it was written.