If prayer is, as we have seen, the central function of the new life of faith, the very heart-beat of our life in God, it is obvious that our prayer life must become the target against which Satan directs his best and most numerous darts. He understands better than we do what prayer means to ourselves and to others. That is why his chief attack is directed against our prayer life. If he can in one way or in another weaken it, his prospects of stealing our life in God without us even noticing it are of the very best.
Ole Hallesby. Prayer: Expanded Edition (Kindle Locations 851-854).