This is an excellent question. Donne was a very learned man, and so the often highly intellectual nature of his poetry inevitably reflects his deep and broad learning. He was well read in numerous subjects, and so it is not surprising that so many of his poems show such a wide range of intellectual interests. He was particularly interested in matters of religion, partly because he was raised in a Catholic family during a time when it was technically illegal to be a Catholic. Eventually, of course, he became one of the most highly respected priests in the Protestant Anglican church. His religious poetry, therefore, often expresses a deeply felt personal desire to think and speak truly about God. "Satire III," for instance, is a splendid example of Donne's very serious commitment to discovering truth in religious matters. At the same time, the poem also shows that Donne knew how difficult it could be to arrive at such truth, especially during times of great religious controversy, such as the era in which he lived. It is possible to argue that Donne's deep interest in religion affected nearly all his poems, including the ones that might seem at first to have little to do with religion in any overt sense. "The Flea," for instance, can seem at first merely an erotic poem of secular love, but it is possible to argue that this poem -- like much of Donne's apparently secular poetry -- reflects, through irony, Donne's commitment to fundamental Christian ideals. In other words, Donne may not intend us to take the speakers of such poems as "The Flea" nearly as seriously as the speakers take themselves. Such speakers, one can argue, are presented ironically in order to teach spiritual lessons through clever indirection. Only a man as learned and as well read and as interested in religious topics as Donne was could have written the kind of poetry Donne composed. An excellent book on this topic is John Donne: Conservative Revolutionary, by N. J. C. Andreasen.