[Our] goal, as the apostle says, is eternal life. “As a reward you have your sanctification and your goal is eternal life “(Rom. 6:22). Our objective is purity of heart, which he so justly describes as sanctification, for without this the goal cannot be reached… Therefore, we must follow completely anything that can bring us to this objective, to this purity of heart, and anything which pulls us away from it must be avoided as being dangerous and damaging. After all, it is for the sake of this that we undertake all that we do and all that we endure. For its sake we hold family, country, honors, riches, the delight of this world and indeed all pleasure in low esteem, and we do so always so as to hold on to purity of heart. With this as our continuous aim, all our acts and thoughts are fully turned toward its achievement, and if it were not ever firmly before our eyes all our efforts would be empty, hesitant, futile, and wasted, and all the thoughts within us would be varied and at loggerheads with one another. /…/

      Everything we do, our every objective, must be undertaken for the sake of this purity of heart. This is why we take on loneliness, fasting, vigils, work, nakedness. For this we must practice reading of the Scripture, together with ail the other virtuous activities, and we do so to trap and to hold our hearts free of the harm of every dangerous passion and in order to rise step by step to the high point of love.

Saint John Cassian (+c. 435)