Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new. 2 Corinthians 5:17

What is meant by the Christian being a new creature? Three thoughts seem to me to spring up from the words, and the first is, the believer must then have been the subject of a radical change. He is said to be a new creature, which is of all things a most sweeping change. There are many changes which a man may undergo, but they may be far from being radical enough to be worth calling a new creation. Saul is among the prophets: hear how he prophesies; if they speak with sacred rapture the secrets of God, so doth he. Is not Saul converted-the Scripture tells us that God gave him another heart! Ay, another heart, but not a new heart. A man may be changed from one sin to another, from reckless profanity to mocking formality, from daring sin to hypocritical pretension to virtue; but such a change as is very far from being saving, and not at all like the work which is called a new creation. Ahab went and humbled himself after his murder of Naboth, and God turned away His vengeance for awhile from him, but that temporary humiliation of Ahab was no sign of a renewal of his nature; it was like the changes of the sea, which today is smooth, but which anon will be as ravenous after wrecks as ever, being still unchanged in its nature, still voracious and cruel, fickle and unstable. Ahab may humble himself, but he is Ahab still, and as Ahab will he go down into the pit.

No doubt there are great moral changes wrought in many which are not saving. I have seen a drunkard become sober; I have known persons of debauched habits become regular; and yet their changes have not amounted to regeneration or the new birth. ..unless the change shall go far deeper than the outward life, he is not saved-he is not a new creature, and, therefore, he is not in Christ Jesus…Do you see, then, how very searching the word is here, a “a new creature,” absolutely a new creation. It is a root and branch change; not an alteration of the walls only, but of the foundation; not a new figuring of the visible tapestry, but a renewal of the fabric itself. Regeneration is a change of the entire nature from top to bottom in all senses and respects. Such is the new birth, such is it to be in Christ and to be renewed by the Holy Ghost.  ~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0881.cfm