Islam took over areas once Christian, but Christianity left its mark on the conquerors.  Jenkins writes:

“No worthwhile history of Islam could omit the history of the Sufi orders, whose practices often recall the bygone world of the Christian monks.  It was the Christian monastics who had sought ecstasy and unity with the divine by the ceaseless repetition of prayers, a practice that would become central to the Sufi tradition.  Once dead, Sufi adepts continued to attract devotees to their tombs, so that venerated sheikhs fulfilled exactly the same role of Muslims that the Christian saints had in their day.”  Elsewhere he notes that Muslims learned the practice of liturgical prostrations from Christians, and suggests that Ramadan was built on the earlier practice of Lent.