Check out how much the speaker packs into those first three words. "Batter" gives us the violence and warfare angle, but "heart" is an even more interesting word choice. If the speaker's just gunning for eternal salvation, you'd think he'd say, "Batter my soul, three-person'd God," because it's the soul that really needs to get turned around in order for him to get in to Heaven. What's the heart got to do with it?
Well, we admit that "heart" can be a metaphor for "soul," but there's so much more to it. The heart, of course, is actually part of your body, and in the context of love, "heart" tends to have a more down-to-earth connotation than the more spiritual "soul." When the speaker uses "heart," then, he shows us quickly that this poem isn't just going to be about a spiritual conversion. This is a physical, corporeal kind of love.
Yet dearly I love you, and would be loved fain, (line 9)
Here, at the turn, the speaker finally admits his love, but that second clause is a bit odd. He says he would be happy if God loves him, but don't you think it would make more sense to say, "Yet dearly I love you, and hope you love me"? Isn't it a little presumptuous to say, "Sure, I'd be happy to be loved - lay it on me"?
But am betroth'd unto your enemy; Divorce me, untie, or break that knot again, (lines 10-11)
There's a lot in these two lines, but from the love angle, we'll just point out that a betrothal is an unusual metaphor for the speaker, since people don't often get engaged without mutual consent. You'd think the speaker could say "captured" or something else to suggest that he is conned into a deal with Satan. As it is, this is a big admission – he willingly falls into the enemy's hands. It's also interesting that, at some point, he agrees to marriage with the Devil, but he doesn't really agree to marriage with God, so much as to a different kind of encounter.
Take me to you, imprison me, for I, Except you enthrall me, never shall be free, Nor ever chaste, except you ravish me. (lines 12-14)
Considering what we saw in the "Quote" above (lines 10-11), it's interesting that, at some point, the speaker agrees to marry the Satan, while here, he doesn't agree to marriage with God so much as a more sexual encounter. Shouldn't Satan be the one who "imprisons?"