It seems like a lot of John Donne's poems are about love. This, too, also seems like a love poem. The way he chooses his words and "talks" to God is in a very private way- the way two people involved in a relationship would speak to eachother, or the way someone who is completely infatuated would speak. An example of this would be lines 12-14:
Take me to you, inprison me, for I,
Except you enthrall me, never shall be free,
Nor ever chaste, except you ravish me.
Donne wants God to be his "one and only." He wants to focus his life on his God, and that is what is most important to him.
He also speaks of the devil and how he tries to take over. The speaker says that he is "betrothed unto your enemy." By this, as I understand it, he is saying that he has sinned. He has done things that he knew were wrong, and is now asking for God's forgiveness: Divorce me, untie or break that knot again...
John Donne wants to be relieved of his sins, and wants to go back to living his life with God.