John Cassian, The Institutes IV.38: Of the renunciant's preparation against temptation, and of the few who are worthy of imitation

Of the renunciant’s preparation against temptation, and of the few who are worthy of imitation.

Wherefore, as Scripture says, “when you go forth to serve the Lord stand in the fear of the Lord, and prepare your mind” not for repose or carelessness or delights, but for temptations and troubles. For “through much tribulation we must enter into the kingdom of God.” For “strait is the gate and narrow is the way which leadeth unto life, and few there be which find it.” Consider therefore that you belong to the few and elect; and do not grow cold after the examples of the lukewarmness of many: but live as the few, that with the few you may be worthy of a place in the kingdom of God: for “many are called, but few chosen,” and it is a “little flock to which it is the Father’s good pleasure to give” an inheritance. You should therefore realize that it is no light sin for one who has made profession of perfection to follow after what is imperfect. And to this state of perfection you may attain by the following steps and in the following way.