Saint Seraphim of Sarov. "On Acquisition of the Holy Spirit".
"And I must further explain, your Godliness, the difference between the operations of the Holy Spirit Who dwells mystically in the hearts of those who believe in our Lord God and Savior Jesus Christ and the operations of the darkness of sin which at the suggestion and instigation of the devil, acts predatorily in us. The Spirit of God reminds us of the words of our Lord Jesus Christ and always acts triumphantly with Him, gladdening our hearts and guiding our steps into the way of peace, while the false, diabolical spirit reasons in the opposite way to Christ, and its actions in us are rebellious, stubborn, and full of the lust of the flesh, the lust of the eyes and the pride of life.
"And whoever lives and believes in Me will never die" (John 11:26). He who has the grace of the Holy Spirit in reward for right faith in Christ, even if on account of human frailty his soul were to die for some sin or other, yet he will not die for ever, but he will be raised by the grace of our Lord Jesus Christ "Who takes away the sin of the world" (John 1:29), and freely gives grace upon grace. Of this grace, which was manifested to the whole world and to our human race by the God-man, it is said in the Gospel: "In Him was life, and the life was the light of men" (John 1:4); and further: "And the light shines in the darkness; and the darkness has never swallowed it" (John 1:5). This means that the grace of the Holy Spirit which is granted at baptism in the name of the Father and the Son and the Holy Spirit, in spite of man's fall into sin, in spite of the darkness surrounding our soul, nevertheless shines in our hearts with the divine light (which has existed from time immemorial) of the inestimable merits of Christ. In the event of a sinner's impenitence this light of Christ cries to the Father: "Abba, Father! Be not angry with this impenitence to the end (of his life)." Then, at the sinners conversion to the way of repentance, it effaces completely all trace of past sin and clothes the former sinner once more in a robe of incorruption spun from the grace of the Holy Spirit. The acquisition of this is the aim of the Christian life, which I have been explaining to your Godliness.
"I will tell you something else, so that you may understand more clearly what is meant by the grace of God, how to recognize it and how its action is manifested particularly in those who are enlightened by it. The grace of the Holy Spirit is the light which enlightens man. The whole of Sacred Scripture speaks about this. Thus our Holy Father David said: "Thy law is a lamp to my feet, and a light to my paths" (Ps. 118:105), and "Unless Thy law had been my meditation, I should have died in my humiliation" (Ps. 118:92). In other words, the grace of the Holy Spirit which is expressed in the Law, by the words of the Lord's commandments, is my lamp and light. If this grace of the Holy Spirit (which I try to acquire so carefully and zealously that I meditate on Thy just judgments seven times a day) did not enlighten me amidst the darkness of the cares which are inseparable from the high calling of my royal rank, whence should I get a spark of light to illumine my way on the path of life, which is darkened by the ill-will of my enemies?
"In fact the Lord has frequently demonstrated before many witnesses how the grace of the Holy Spirit acts on people whom He has sanctified and illumined by His great inspirations. Remember Moses after his talk with God on Mount Sinai. He so shone with an extraordinary light that people were unable to look at him. He was even forced to wear a veil when he appeared in public. Remember the Transfiguration of the Lord on Mount Tabor. A great light encircled Him, "and His raiment became shining, exceedingly white like snow" (Mk. 9:3), and His disciples fell on their faces from fear. But when Moses and Elijah appeared to Him in that light, a cloud overshadowed them in order to hide the radiance of the light of the divine grace which blinded the eyes of the disciples. Thus the grace of the All-Holy Spirit of God appears in an ineffable light to all to whom God reveals its action."