Ps 50:12

“"The one who says, I know him, and does not keep his commandments is a liar, and the truth is not in him. But whoever keeps his word, in him the love of God has been truly perfected. By this we know that we are in him; he who says that he abides in him ought himself also to walk as he walked." My reason for telling you, little children, that everyone who is born of God does not sin, is that you should not sin and that you should know that as long as you do not sin you abide in the birth that God has given you. Truly, they who abide in that birth cannot sin. "For what does light have in common with darkness? Or Christ with Belial?" As day is distinct from night, so righteousness and unrighteousness, sin and good works, Christ and Antichrist cannot blend. If we give Christ a lodging place in our hearts, we banish the devil therefrom. If we sin and the devil enters through the gate of sin, Christ will immediately withdraw. Hence David after sinning says, "Restore to me the joy of your salvation," that is, the joy that he had lost by sinning.”

“"Give back to me the exultation of Your salvation" [Psalm 51:12]. "Give back" what I had; what by sinning I had lost: to wit, of Your Christ. For who without Him can be made whole? Because even before that He was Son of Mary, "In the beginning He was the Word, and the Word was with God, and the Word was God;" [John 1:1] and so, by the holy fathers a future dispensation of flesh taken upon Him, was looked for; as is believed by us to have been done. Times are changed, not faith. "And with Principal Spirit confirm me." Some have here understood the Trinity in God, Itself God; the dispensation of Flesh being excepted therefrom: since it is written, "God is a Spirit." [John 4:24] For that which is not body, and yet is, seems to exist in such sort as that it is spirit. Therefore some understand here the Trinity spoken of: "In upright Spirit," the Son; in "Holy Spirit," Holy Ghost; in "Principal Spirit," Father. It is not any heretical opinion, therefore, whether this be so, or whether "upright Spirit" He would have to be taken of man himself (when He says, "An upright spirit renew in my inner parts"), which I have bowed and distorted by sinning, so that in that case the Holy Spirit be Himself the Principal Spirit: which also he would not have to be taken away from him, and thereby would have himself to be confirmed therein.”

“"Restore to me": for grace does two things in a person. One is in respect to higher things, namely, it gives delight, because he who has grace has charity, and he who has charity loves God and possesses him; and he who possesses what he loves rejoices. Therefore, where there is charity, there is joy. Rom. 14: "The kingdom of God is not food and drink, but joy in the Holy Spirit." The Psalmist had lost this joy; and therefore he asks for it to be restored to him when he says, "Restore to me the joy" -- not of worldly things, but "of your salvation," that is, of your saving act. Another text has, "the joy of Jesus," namely of the Savior, through whom the remission of sins is accomplished. Hab. 3: "I will exult in God my Jesus." The other effect is in respect to lower things; and this effect is confirmation in grace, which is accomplished through the Holy Spirit: "and confirm me with a sovereign spirit." The Holy Spirit strengthens in two ways: in one way, against evils. Is. 8: "With a strong hand he instructed me." In another way, in good. Is. 40: "Those who hope in the Lord shall renew their strength." This strength is had through the Holy Spirit; for the body is not firm for enduring or for acting except through the strength of spirits; so also a person is not strong without the Holy Spirit. But he would not provide strength unless he were a sovereign spirit; because an inferior power is not sufficient to provide aid against a superior one. But the power of the Devil is great. Job 41: "There is no power on earth that can be compared to him." Therefore, against the Devil, a person needs to be aided by a sovereign spirit, namely one that is princely and has dominion over all things. And a person needs this spirit against the spirit of the flesh. Num. 16: "Most mighty God, God of the spirits of all flesh." Also, against the spirit of the world. 1 Cor. 2: "But we have not received the spirit of this world, but the Spirit who is from God." Also, against the spirit of the Devil. 1 Sam. 18: "An evil spirit of the Lord came upon Saul." It should be noted that in this reading there is a threefold mention of "spirit": for there is mentioned a right spirit, a holy spirit, and a sovereign spirit. And according to the Gloss, some take "spirit" as said essentially, insofar as everything that is not a body is a spirit. Hence the Father and the Son and the Holy Spirit are called spirit. But it is better to take it personally. Now the Holy Spirit does three things in a person. First, rectitude of intention. Ps. 142: "Your good spirit," etc. Also, he sanctifies us. Rom. 1: "According to the spirit of sanctification." Also, he ennobles us and makes us princes. Gal. 4: "Because you are sons of God, God has sent the Spirit of his Son into your hearts."”