The starting point for analysis is the title and the notion of a "relic," a physical object associated normally with a saint in the Roman Catholic Church, and set in a shrine (often an alcove within a church) where people can address prayers to that saint. The bracelet of bright hair, belonging to the narrator's beloved, is thus being compared to a relic.
John Donne himself, though, was not a Roman Catholic, but a member of (and eventually priest in) the Church of England, a Protestant Church in which veneration of relics and prayers to saints were considered a form of idolatry. The notion of becoming relics in the future which might work miracles thus is rather theologically ambiguous, as is the casting of the woman as Mary Magdalene, a woman in the Bible who was understood in Donne's time as having a double history of sexual misconduct and eventual salvation by Jesus, becoming one of his followers.
The relationship of the two lovers, which is chaste, and description of her as a secret miracle suggests that the poem is emphasizing the salvation of Mary Magdalene and the good or even miraculous influence of the woman on the narrator's life, a goodness that even transcends death itself.