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.