“There is therefore now no condemnation
to those who are in Christ Jesus…”

This verse is often quoted, but most often it stops there. Read the rest of the verse:

“…who do not walk according to the flesh,
but according to the Spirit.”
Romans 8:1

It’s not difficult to discern those who are in Christ Jesus, versus those who walk in the flesh. When one is “born again,” he is so filled with the Spirit of God that it’s all he thinks of, and everything in his life centers around the love of the things of God, not the love of religion, but love of truth. This is not by effort, it’s not even by the person’s will, but it is the natural manifestation of the Spirit of God within.

“For those who live according to the flesh
SET THEIR MINDS on the things of the flesh,
but those who live according to the Spirit,
the things of the Spirit.”
Romans 8:5

So if a person does not have this love within him, he is not born again but instead is “religious.” He does not belong to Christ. Religion is man’s effort to appease God. The born-again experience is man’s heart response to God.

“Do not marvel that I said to you, ‘You MUST be born again.”
John 3:7

“Now if anyone does not have the Spirit of Christ,
he is not His.”
Romans 8:9

One filled with the Spirit of Christ will always be in agreement with the Word of God. Those who profess to be “Christians” but who deny the Word of God – they are not his.

More:
What does “born again” mean?