The brothers asked Agatho, “Abba, which virtue in our way of life needs most effort to acquire?” He said to them, “I may be wrong but I think nothing needs so much effort as prayer to God. If anyone wants to pray, the demons try to interrupt the prayer, for they know that prayer is the only thing that hinders them. All the other efforts in a religious life, whether they are made vehemently or gently, have room for a measure of rest. But we need to pray till our dying breath. That is the great struggle.”

-from The Desert Fathers: Sayings of the Early Christian Monks