Can One’s Salvation Be Given Up?

There's no reason to.
I'm even in the priesthood myself now.

5you also, like living stones, are being built into a spiritual house to be a holy priesthood, offering spiritual sacrifices acceptable to God through Jesus Christ. - 1 Peter 2:5

And just like under the old system, my sacrifices are not acceptable if I harbor discord, hatred, and contempt for others in my heart when making those sacrifices, though I may think they are acceptable. Most mean Christians do think their sacrifices are acceptable to Jesus.
But that's not what Jesus says.

Correct! So you don't obey Jesus and His words to the Jews. You're coming around.

Jews don't have to literally fulfill the various worship requirements that are fulfilled in Christ either. There is no special, different kingdom for them. We are together one body, one kingdom and nation of God's people.

We are now, but it wasn't that way before the cross. The disciples were forbidden to take the message of the kingdom to the Gentiles and Samaritans. Paul's message is for everyone, Jew and Gentile to make one body.

We must be careful to remember that all scriptures must harmonize in order to understand the doctrine that Jesus taught.

I agree that scripture is harmonized, but we cannot take specific passages and apply them erroneously to make them say something they do not.

Jesus wants the same thing for the Jewish people that He wants for the rest of us; He wants them to become born again Christians.

But you do agree that, during the 4 gospels, they had to do those things since Christ have not died on the cross?

Reconciliation with God is the main issue. The Jews were saved when they believed they were in need of a Savior, and that God would send one. We believe we need one, and that God sent Him.

Peter preached the death of Jesus as a murder indictment to the Jews (whom ye have crucified), and the resurrection as proof to all the Jews that Jesus was indeed the Son of God and their promised Messiah (both Lord and Christ) (Acts 2:36)

That verse is the critical part of the sermon he preached that pricked the hearts of all the Jewish listeners so its worth highlighting here, for all of us to read slowly and digest:

Therefore let all the house of Israel (Notice Peter was not concerned about the Gentiles at all here) know assuredly, that God hath made the same Jesus, whom ye have crucified, both Lord and Christ.

That is the same message that form the basis of the gospel of the Kingdom. (John 20:31)

Only because it was to go to the Jew first.
Any gentile could join himself to the nation of Israel at that time and before.

But I think the issue for you is you think Jesus' message sounds like a works gospel, so you have concluded that he taught another kingdom message than Paul.

I get the concept, but there are many issues I take exception with. In all honestly I really don’t feeling like going down this road again.

Absolutely.
No question about it.

So John146's point was that you are also picking and choosing which instructions that Jesus gave to the Jews, during the 4 Gospels, are valid or invalid for us now.

Yep, it will take time for people to realize the distinction between what Peter was preaching to the Jews during early Acts, and what Paul received as revelations from the ascended Christ himself, after he was saved.

No, I don't.
I go by what the Bible itself teaches about that.

The Jews were saved in early Acts by what Peter told them to do in Acts 2:38 "Repent and be baptized".

The idea of repentance is for the Jews and can be viewed from the parable of the tenants, told in all 3 synoptic gospels. (Matthew 21:33-46; Mark 12:1-12; Luke 20:9-19)

When Jesus and the 12 were preaching from Matt-John, they need to repent of rejecting God their Father in the OT, and believe in his Son is their promised King and Messiah, as foretold by their prophets.

But as Jesus told them in the parable of the tenants, they were wicked and decided to slay the son instead, thinking they can then take control of the vineyard. The words they told Pilate was really horrific from this perspective

Matthew 27:25 Then answered all the people, and said, His blood be on us, and on our children.

When Jesus was on the cross, he asked his father to forgive them for this act, because they know not what they do, the Father agreed to Jesus request.

So when Peter urged his Jewish brothers at Pentecost to repent (Acts 2:38), they are to repent from their horrific act of murdering the Son of God, and accept him once again as the Son of God and their King.

Everyone here is believing they are doing that.

The truth is that all of us are interpreting scripture based on certain preconceived beliefs that we have been taught before and consider them unquestionable. These are called axioms.

A gentile could join himself to the nation of Israel if he wanted to. But clearly the Jews themselves were in the majority. There was no reason to even acknowledge the presence of the observant gentiles among them.

Not me.
Really.
Hebrews, especially, taught me everything I know about the time of reformation from the old covenant to the New Covenants.

I mean, truth is, I learned a lot from talking to Messianic believers in on-line forums.