Through God's precious gift of Baptism we enter into new life, being born again (see e.g. John 3:3-5), having therefore the very promise of the Spirit given to us--which is right there in the text of Acts 2:38, our lives are new, brand new, in Christ. For, the Apostle St. Paul will say elsewhere that when we were baptized we died with Christ, were buried with Christ, and raised together with Christ to new life, to share in the life of Christ. See how in Romans 6 the Apostle has rebuked the wrongful thinking of "Since grace abounds, should we go on sinning?" (Romans 6:1) instead, on account of our baptism, and all that this means, we are now to reckon ourselves dead to sin and alive to God, as slaves of righteousness.
Therefore the ever-present call to metanoia, repentance, is there that we should "not be conformed to the pattern of this age, but be transformed by the renewing of your mind" (Romans 12:2). This word, metanoia, quite literally speaks of our minds being changed, a restructuring and re-ordering of how we live, how we see things, how we think about things. Paul, will, again, call us to this drastic transformation and change in our thinking when he, in Philippians 2, writes, "Have this same mind in you that was in Christ Jesus". Now Paul uses a different word here in Philippians 2:5 than he does in Romans 12:2, in Romans 12:2 Paul uses the ordinary word for "mind" in Greek, nous; but here in Philippians 2:5 the Apostle uses the verb phroneiotho, a form of the vern phroneo: to think, to feel, be mindful, to discern, to judge. It speaks of an entire orientation of thought and feeling; to therefore think, feel--in what way? As Christ Himself who humbly emptied Himself and came as a slave, obedient to the Father, offering Himself even to death on the cross with all of its shame.
Repentance does include our confession of our sin--something we are always called to do, for St. John in his epistle will say, "If we say we have no sin, the truth is not in us" and continues "But if we confess our sin, He will forgive us and cleans us from all unrighteousness". In this life we will struggle against the old man within us, the passions and lusts of our flesh, as the Apostle St. Paul talks about in Romans 7. We continue to struggle, the old man vs the new man. So we confess, we freely confess our sin--for God freely declares us forgiven and justified on Christ's account. This is what is meant that where sin abounds, grace abounds all the more.
But also included here in repentance is not merely our messing up and confessing our sin; but the continued call of Christ on our lives, to be converted and have our hearts, our minds, our attitudes adjusted toward righteousness, toward holiness, toward the image and likeness of Jesus Christ. This is the work and power of the Holy Spirit continually operating in our lives, that "He who began a good work in you will continue that good work until the day of Christ Jesus" (Philippians 1:6). Not that we shall attain glory in this life, for Paul also writes,
"not that I have already obtained this or am already complete, but I press on to make it my own, because Christ Jesus has made me His own. Brothers I do not consider that I have made it my own. But one thing I do: forgetting what lies behind and straining forward to what lies ahead, I press on toward the goal for the prize of the upward call of God in Christ Jesus." (Philippians 3:12-14
This metanoia is part of the cross of discipleship with Christ our Lord has called us to take up upon ourselves. To strive forward, to follow Him, to live in accordance with His way of doing things, and holding to His way of thinking, to act as He acts, to live as He lives, to imitate Him, to follow Him.
So when the great invitation is given, "Repent and be baptized" it is an all-comprehensive invitation, to come and receive the new life that is in Christ, through Baptism receiving new life in Christ and to the onward, forward journey of Christian discipleship. The way is hard, the journey is fraught with many perils--this Christian life is a cross, a harsh and painful cross. But the cross shall become a joy to us, not through our own ability, strength, or can-do-man-ship; but the Holy Spirit shall abide in us and with us, and He shall be our Comfort. And as we are called to life together, as the Church, abiding in the Lord Jesus, as God's children, being sustained by the life-giving word of God, by His grace, as we hear the precious word of God preached, as we receive God's abundant mercies in His Sacraments, as we live and abide, and have life together in this community and communion of holiness and faith, God is making us what He wills for us to be. And we are not alone, God is with us; and as He gives Himself for us in the Church, we come together and give ourselves to one another in the Church. This is what the Scriptures call Koinonia, fellowship.
-CryptoLUtheran