Violent imagery in a holy sonnet: Batter my heart, three-personed God.
In John Donne's "Batter my heart, three-personed God ," the speaker is in despair about his ability to be chaste. He knows that he loves God and wants to be loved in return. But he is too weak to resist the pull of evil. He knows that his reason should be his defense against sin; in fact, he calls it God's "viceroy" in him. A viceroy is a governor appointed by a king to rule in his place in a far away colony. The metaphor of reason as a viceroy is a way of saying that God is represented in the speaker by his ability to reason. Reason tells him he is sinful, "but it is captived," or in other words, the speaker's reason itself is held hostage by evil and can't help him.
So the speaker asks God to invade him. Before he can be free of sin, he's going to have to captured and ravished by God. He compares himself through simile to a walled town. This is a conceit, or complicated comparison that is carried throughout the poem. He asks God to use a battering ram to knock down the gate to the town, his heart. Then, he asks God to use force to "break, blow, burn" the town. The town is the speaker, but he says that he is already "usurped," because someone else already holds him hostage. That somebody is sin; his "town" is already occupied by the enemy ("to another due"). In a way, he is asking God to free him from his lust, his desire not to be good, not to be chaste. He says that force is the only way to make this happen, since reason is not strong enough to protect him, and since he does not have the strength to free himself from sin.
Besides comparing himself to a town under siege, the speaker is a man connected by a kind of marriage to the enemy. He loves God "dearly," but is "betrothed unto your enemy." The enemy is sin or evil or Satan. The betrothal is also referred to as a "knot" that he asks God to "untie or break." He wants God to "divorce" him from the enemy.