Is the “new creation” immediately completed or a gradual change?
While there is a traditional belief that one is 'born again' at baptism, there are several problems with this as scripture shows. I will outline another way to view being 'born again', but first as it impacts on the Q.
We are saved by our acceptance of Jesus' payment for our sin and removal of the death penalty and are now seen by the Father as righteous through His glorious son and his righteousness applied to us in grace.
Through the living Christ we may join the new creation - 'that person is part of the new creation' - which has been going on since Jesus ushered it in as he was raised from the dead.
This process, began 2000 yrs ago, is not yet finished. We are not yet finished.
When God created man in His image, it is not possible for this to be true in reality until it is accomplished in Christ - and then, only at our change to immortality at his return. This too is work in progress.
1 Cor 15:49 ‘Just as we have borne the image of the earthy, we will also bear the image of the heavenly’
We can think of God creating all things by His word - this does not need to imply that He is finished creating. He is Creator - and continues to create. In us, He is creating holy righteous character - only in and through His son Jesus. Only when this process is finished, are we made in His image.
1 Pet 3:19 For Christ also suffered once for sins ... that he might bring us to God, being put to death in the flesh but made alive in the spirit.
We are given the spirit as a down-payment, or as the NASB puts it, 'pledge'. This is our entry into the new life with God's gift of His spirit at baptism. We are not yet born again, or 'born from above' until we are no longer flesh and blood - just as it was for Jesus.
2 Cor 1:21 Now He who establishes us with you in Christ and anointed us is God, 22 who also sealed us and gave us the Spirit in our hearts as a pledge.
Notice two things here - we are 'established', and 'sealed', set apart, God is actively working in us in a way that He wasn't before. We are His, but for us it is still by faith, faith and a hope not yet realised.
Rom 8:25 But if we hope for what we do not see, with perseverance we wait eagerly for it.
Romans 8:23 Not only that, but we ourselves, who have the first-fruits of the Spirit, groan inwardly as we wait eagerly for our adoption as sons, the redemption of our bodies.
2 Corinthians 5:5 And God has prepared us for this very purpose and has given us the Spirit as a pledge of what is to come.
Ephesians 1:14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
We are set aside to be inheritors with Christ - we have not yet taken possession of the glory that awaits us at the end of this age when we are 'born from above' by God, of the spirit.
At this stage we are begotten - just as one is conceived by a man and woman, but not yet born. The process is underway and God is in charge of bringing us to fruition - by nothing less than His sovereign will.
Jesus is the firstborn of many brethren Rom 8:29 When did he become so? At his resurrection to life in the spirit.
Some other verses that show when we are 'born from above'.
John 3:3 Truly, truly, I say to you, unless one is born again (or from above) he cannot see the kingdom of God.
1 Cor 15:50 flesh and blood cannot inherit the kingdom of God. If we are still flesh and blood - we have not yet (fully) entered.
John 3:5 no one can enter the kingdom of God unless he is born of water and the Spirit.
So the new creation has begun - it began before Adam, before the garden, but is realised in Christ, and brought to fruition at our change. In the meantime, we grow, mature, overcome, live faithful lives in hope and rest knowing we are in good hands until finally delivered into the kingdom.