"Therefore if anyone is in Christ, this person is a new creation; the old things passed away; behold, new things have come."

So what are the old things that passed away?

I believe this is what passed away:

The Old Covenant system, including the Law of Moses, temple rituals, priesthood, sacrifices, and national exclusivity. Hebrews 8:13 confirms this:

"When He said, 'A new covenant,' He has made the first obsolete. But whatever is becoming obsolete and growing old is about to disappear."

The old identity based on flesh, lineage, and external righteousness. Paul had just said in 2 Corinthians 5:16:

"Therefore from now on we recognize no one by the flesh..."

That means the old ways of identifying people, by Jewish descent, law-keeping, or physical circumcision, had passed.

Life under condemnation and death, which was the ministry of the Law (2 Corinthians 3:6-9). The Law could not give life; it only condemned.

The Adamic state, which brought spiritual death and alienation. In Christ, a new humanity is born. Romans 5:12-19 ties this transformation from Adam to Christ.

In short:

The old covenant, old identity, old system of law and death passed away. In Christ, a new covenant, new creation, and spiritual identity arrived.