I think that it is actually more accurate to say that Montag is trying to memorize something on the subway instead of saying that he is trying to remember. What he is trying to memorize is a passage from the Gospel of Matthew.
He is trying to memorize a bit from Matthew 6. This is the part where Jesus is telling people not to worry about stuff on earth -- they should trust in God to take care of them. Jesus says
And why take ye thought for raiment? Consider the lilies of the field, how they grow; they toil not, neither do they spin
But Montag cannot memorize this because the ad for Denham's Dentrifice keeps blaring away. This symbolizes how the society never lets anyone have time to think about anything other than material stuff.
At the beginning of Part II, Montag travels to Faber's home and takes the subway. While Montag is on the train, he attempts to remember the Bible verse Matthew 6:28, which says, "And why take ye thought for raiment? Consider the lilies of the field, how they grow; they toil not, neither do they spin" (King James Version). Unfortunately, Montag cannot remember the enter verse because he is continually interrupted by the extremely loud Denham's Dentifrice advertisement blaring through the speakers on the train. Montag compares his ability to remember the Bible verse to sand going through a sieve. Montag is only able to recall the words "Consider the lilies of the field" because the advertisement is so distracting. Montag becomes extremely frustrated with his inability to remember the entire verse and cries aloud on the train. The passengers notice that Montag is acting strange, and he runs off the train when it finally comes to a stop.
Guy Montag is not trying to remember something he is trying to memorize the bible as he is reading it but the commercial that is play is so loud and annoying it is confusing him.