How we cite our quotes:
O, beware, my lord, of jealousy;
It is the green-eyed monster which doth mock
The meat it feeds on; (3.3.15)
Iago's pretty good at manipulating Othello, don't you think? Here, he pretends to warn Othello not to be a jealous man, pointing out that jealousy ends up destroying the heart of the man who falls prey to it.
Why, why is this?
Think'st thou I'ld make a life of jealousy,
To follow still the changes of the moon
With fresh suspicions? No; to be once in doubt
Is once to be resolved: exchange me for a goat,
When I shall turn the business of my soul
To such exsufflicate and blown surmises,
Matching thy inference. 'Tis not to make me jealous
To say my wife is fair, feeds well, loves company,
Is free of speech, sings, plays and dances well;
Where virtue is, these are more virtuous:
Nor from mine own weak merits will I draw
The smallest fear or doubt of her revolt;
For she had eyes, and chose me. No, Iago;
I'll see before I doubt; when I doubt, prove;
And on the proof, there is no more but this,—
Away at once with love or jealousy! (3.3.31)
Here, Othello claims that he won't be destroyed by jealousy. He reasons that Desdemona "had eyes, and chose [him]" despite, presumably, the fact that he is black. But, then, Othello lets slip that he may in fact be a bit more jealous and suspicious of his wife than he lets on – he says he wants some "proof" of Desdemona's infidelity. Looks like Iago's master plan may work out after all.
Trifles light as air
Are to the jealous confirmations strong
As proofs of holy writ: this may do something. (3.3.33)
Iago realizes that real proof of Desdemona's supposed infidelity is not necessary because mere suspicion is enough to feed Othello's jealousy. In the case of Othello, Iago will use the handkerchief Othello gave Desdemona in order to convince Othello that Desdemona's been cheating. (Remember, when Desdemona dropped her handkerchief by accident, Emilia picked it up and gave it to Iago. Iago says he's going to drop it for Cassio to find.) Even though the handkerchief is a mere "trifle, light as air," once Othello sees it in another man's possession, he'll think he has solid proof that Desdemona is unfaithful. When Iago says the handkerchief will be "as proofs of holy writ" to Othello, he means that Othello will see the handkerchief as the gospel truth that Desdemona's a cheater.