How is a person assured they have this circumcision?
That's a great question.
In looking at the text, one ought to conclude that a person should feel complete. A person ought to also feel a significant psychological change, as the driving factor for sin has been removed and disassociated from that person's spirit/heart. Just as Adam and Eve felt the effects of their added Sinful Nature, we should feel the opposite, which is the release of those effects. This is also known as being held captive by Satan to do his will. Jesus referred to this relationship as father and son . . . scary.
The Scripture tells us that the water baptism reflected the death and buriel with and in Christ, that a person is raised to New Life. The text states that a person transfers from death to life from this Spiritual Circumcision, and this is the release from "prison," or "Babylon," or "Egypt."
Leviticus 26:13 NLT - "I am the LORD your God, who brought you out of the land of Egypt so you would no longer be their slaves. I broke the yoke of slavery from your neck so you can walk with your heads held high."
We are also told that victory over Satan has been achieved, and this by the Right Hand of Salvation, who is Christ. When this victory has been granted, we should feel the release of this unholy prison. Just as a prisoner is likely thrilled to be released to the streets again, one held prisoner by Satan to do his will should also feel such a release when granted by Christ.
This is what Paul is referencing in Romans 2 as he discussed being Transformed by the renewing of our minds; the change by which we think. We will change. If we are granted the Divine Nature, we WILL change.
2 Peter 1:4 NLT - "And because of his glory and excellence, he has given us great and precious promises. These are the promises that
enable you to share his divine nature and escape the world's corruption caused by human desires."
Another way of discussing the Circumcision of Christ is through discussing the Veil of Moses.
2 Corinthians 3:13-18 NLT - "We are not like Moses, who put a veil over his face so the people of Israel would not see the glory, even though it was destined to fade away. 14 But the people's minds were hardened, and to this day whenever the old covenant is being read, the same veil covers their minds so they cannot understand the truth. And this veil can be removed only by believing in Christ. 15 Yes, even today when they read Moses' writings, their hearts are covered with that veil, and they do not understand. 16 But whenever someone turns to the Lord, the veil is taken away. 17 For the Lord is the Spirit, and wherever the Spirit of the Lord is, there is freedom. 18 So all of us who have had that veil removed can see and reflect the glory of the Lord.
And the Lord--who is the Spirit--makes us more and more like him as we are changed into his glorious image."
Understanding that the removal of the Veil is the same thing as the Spiritual Circumcision of Christ, this should help to tie in several Biblical factors in one full swoop. This stuff is beautiful when it gets into your soul. The scope of the entire Bible is about to change if you embrace these Scriptures.