Shut up, thought Montag. Consider the lilies of the field.

"Denham's Dentifrice."

They toil not--

"Denham's--"

Consider the lilies of the field, shut up, shut up.

"Dentifrice! "
(Bradbury, Fahrenheit 451, Google Books)

The relevant verse reads as follows:

(28) So why do you worry about clothing? Consider the lilies of the field, how they grow: they neither toil nor spin.
(Matthew, New King James Version)

The relevance of this verse to Montag's struggle is that he is trying too hard; he has worked himself into sickness through worrying, and is now trying to memorize a whole book in a matter of minutes. Instead of Montag's emotional flailing, which gets him in trouble with his wife and his superior at the firehouse, he would be better served to sit back and think calmly. The lilies do no work and are clothed beautifully in their petals, which protect and facilitate reproduction; Montag should think before acting and try to protect his books, protecting their potential. Montag is worrying too much about the future when he should be worrying about the present. He is worried about "clothing" -- the larger consequences of his crime from the fireouse -- when he should be taking things slowly, creating a plan of action against the society at large. He has no hope of creating massive change all by himself, but an educated man with a small library of books might cause change to ripple out from himself. Instead, he loses his books and wife, and is almost killed because he reacts with superficial emotion (I want to do this) instead of rational consideration (I should do this because of this).

A major theme in this novel is alienation. Mankind has alienated itself by destroying books, books which contain the record of the history of mankind. When men are alienated, they are lonely and depressed and they wind up finding no reason to live, so they commit suicide, like Mildred. It is significant that Montag decides to memorize something from the Bible, because the Bible is the word of God, given to man so that man will know how to live in the world. The particular book that Montag tries to memorize is Ecclesiastes. The reason that this is significant is that this book is written by a wise philosopher, some believe King Solomon himself. In this book, the author grapples with the meaning of life, man's place in the world, etc. The author experiences everything life has to offer (fame, fortune, family) and finds them all "meaningless, chasing after the wind." The conclusion of the book of Ecclesiastes is found in Chapter 12, verse 13:

The conclusion, when all has been heard, is: fear God and keep His commandments, because this applies to every person.

In Fahrenheit 451, man has strayed far, far away from his purpose, and thus he is totally alienated. But, there is hope - God's word will not return void and can be stored up in a man's heart through memorization, which Montag decides to do to redeem himself.

Ray Bradbury is one of my favorite authors of all time. While he often cringes when people point out his religious themes and motifs, one cannot deny their existence in almost all of his works. He was raised a Baptist so these themes are a part of the fiber of his writing, despite his denials.

Read about it here on eNotes.