In my opinion the grace of the Holy Spirit most readily fills those who undertake spiritual work wholeheartedly and determine from the very beginning to stand firm and never to give ground to the enemy in no matter what battle, until they conquer him. However, the Holy Spirit, Who has called them, at first makes all things easy for them, in order thus to sweeten the beginning of the work of repentance, and only later shows them its ways in their full truth (arduousness). Helping them in all things, He impresses on them what works of repentance they should undertake, and lays down the form and limits both as regards the body and the soul, until He brings them to complete conversion to God, their Creator.
For this purpose He constantly urges them to give exertion to body and soul in order that both alike, being equally sanctified, should equally become worthy heirs of eternal life; to exert the body in constant fasting, work and frequent vigils, and the soul, in spiritual exercises and diligence in all forms of service (and obediences) performed through the body. This (to do nothing carelessly, but always with care and the fear of God) should be zealously observed in all work done with the body, if we wish it to bear fruit.