What is Montag trying to do while on the subway and why?

Montag is trying to read the Bible.... to memorize its words. He wanted to absorb and remember as much as he could before being forced to turn it over to Beatty.

There were people in the suction train but he held the book in his hands and the silly thought came to him, if you read fast and read all, maybe some of the sand will stay in the sieve. But he read and the words fell through, and he thought, in a few hours, there will be Beatty, and here will be me handing this over, so no phrase must escape me, each line must be memorized. I will myself to do it.