In this sonnet the three quatrain divisions each use a unique metaphor to explain the speakers feelings about being a sinner and his pleas to God to help him be a better person.
In the first quatrain he suggests that God is a metal worker who can "batter my heart" to make it new. God will "break, blow, and burn, and make me new."
In the second quatrain, he compares himself to a city that is overtaken by sin. His reason has been taken captive and his not capable of defending him.
In the third quatrain he explains his sinful ways he compares his sinful soul to a bride or groom that is married to sin. He is asking the God grant him a divorce so that, as he states in the final couplet, he can be ravished by the love of God. It is rather shocking language, hence a metaphysical conceit, but like all clever comparisons, all of these work well in his effort to pray for a change from God.
Donne's Sonnet XIV takes the strict form of a sonnet and plays with the form and the meter to help the reader to understand the themes presented. Instead of using a pure iambic pentameter throughout, Donne uses an irregular form of iambic pentameter to allow the reader to understand that this is no ordinary love poem. The speaker in this case is not a man reaching out to a female lover, but instead a speaker who uses a female voice to grapple with his relationship with God.
The irregular meter helps to create the tension felt by the speaker as he struggles with his God. The use of pure iambic pentameter for the final couplet could indicate a sense of peace or religious enlightenment obtained by the speaker at the end of his battle.