"Holy Sonnet 14" is (surprise!) a sonnet. It is the 14th in a series of sonnets John Donne wrote from 1609-1611. These poems are all religious in nature, and deal with themes like death, divine love, and faith.
Coming near the end of this sequence, "Holy Sonnet 14" depicts a speaker's desperate plea to God to return to the speaker's soul. What makes the poem unique and forceful is the way that the speaker frames this plea. The speaker boldly commands God to force his way into the speaker's soul, roughly seducing the speaker in what is an explicit comparison to sexual love.
"Batter my heart, three person'd God," begins the speaker. The phrase "three person'd God" is an allusion to the Holy Trinity—Christianity's depiction of God as composed of three different entities: the Father, the Son (Jesus), and the Holy Spirit. Here, the speaker's referring to the whole Trinity, begging the Trinity to attack the speaker's heart.
More precisely, the speaker orders the Trinity to attack the heart. The order immediately puts the poem in a Christian context. This is going to be a poem about faith, about the speaker's personal relationship with God. Additionally, the word "Batter" suggests the battering rams that armies use to break through the doors of fortresses and city walls. This word choice, then, implicitly compares the speaker's heart to a fortress that must be broken into. This metaphor will come into play throughout the poem.
For now, there are two observations to make about this initial command. First, that it's an instance of apostrophe, an address to a being that cannot respond. Throughout the poem, the speaker addresses God as "you," and God remains silent. This use of apostrophe gives the poem a certain quality—that of prayer. A poem that acts as a prayer or discusses the speaker's relationship with God is called a devotional poem, and that's just what "Holy Sonnet 14" is.
Second, this initial phrase is a command. The speaker is ordering God around. That's a bold thing to do, because in Christianity it's usually the other way around—God commands mortals. Ordinary people are supposed to be humble and penitent, quietly asking for forgiveness. In fact, commanding God is not a very pious, reverent thing to do. At the same time, however, piety is exactly what the speaker's after. The speaker wants to grow more faithful, yet speaks in a manner that is against the tenets of the Christian religion. This is a paradox, a purposefully contradictory pair of gestures.
The speaker will employ paradox throughout the poem. This in itself is nothing out of the ordinary. Renaissance poetry, especially love poetry (and this poem, it will become clear, is a kind of love poem), was obsessed with paradox. Renaissance poets used it to capture the complicated intensity of human passion. As the poem progresses, paradox will play a prominent role.
As is traditional for English-language sonnets, the poem has 14 lines of rhymed iambic pentameter. This simply means that each line has five feet in a "da-DUM" rhythm. Its rhyme scheme is based on that used by Petrarch, with an added hint of Shakespeare—the two most famous sonnet writers in Italian and English.