What is the purpose of the paradox in these lines from the poem? Take me to you, imprison me, for I, Except you enthrall me, never shall be free, Nor ever chaste, except you ravish me.

The answer is:

It emphasizes the poet’s urge for redemption.

The paradoxes are because of the writer and his logic which will only be persuaded that God exists and that He is powerful only when he will be able to see an example of it with his own eyes. The writer desperately desires it so, because he believes it will be the one that will "save" him.