Psalm 51:10, Create in me a clean heart, O God, and renew a right spirit within me.
This ought to be our daily prayer, for we sin daily. The Christian life is none other than to have a daily renewal of the heart. “What does this feel like?” That is an evangelical question, for evangelicals think you have to feel in order to know you have the Holy Spirit or that God is at work in your life. The Scriptures do not point us to rely on our feelings; they point us to rely on Christ.
“So, how do I know if I have a clean heart and a renewed spirit?” It is not something you can see or feel, yet the Holy Spirit daily enables us to resist sin. Luther describes it best:
Thus, a Christian life is nothing else than a daily baptism, begun once and continuing ever after. For we must keep at it without ceasing, always purging whatever pertains to the old Adam, so that whatever belongs to the new creature may come forth. What is the old creature? It is what is born in us from Adam, irascible, spiteful, envious, unchaste, greedy, lazy, proud—yes—and unbelieving; it is beset with all vices and by nature has nothing good in it. Now, when we enter Christ’s kingdom [via Baptism], this corruption must daily decrease so that the longer we live the more gentle, patient, and meek we become [this is what belongs to the new creature in Baptism], and the more we break away from greed, hatred, envy, and pride.LC, “Fourth Part: Concerning Baptism,” 65-67; KW 465
In Baptism, then, God daily renews us to resist sin; and when we give in to temptation, we can approach God with confidence to receive mercy (Hebrews 4:16).