Past sins forgiven

He will bring the people back to the law to save them from sin - Matt 5:18 not an iota, not a dot, will pass from the Law until all is accomplished.

Romans 3:25 KJV
Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;

What is your understanding of Matthew 1:21...and He shall save His people from their sins?

Christ came primarily and initially to the nation of Israel, and had every Jew repented and believe on Him, all Israel would have been saved. But after the Second Coming of Christ "all Israel shall be saved" in that there will be redeemed Jews from every one of the twelve tribes who will see Christ and believe on Him. But only one-third of all Jewry worldwide shall be saved, as it is written.

All sins past, present and future....not just past.

I believe it is accurate to say that the verse is first speaking of the people of Israel.

When you go to Daniel "thy people" is a reference to the Jews. Hence "His people" and "thy people" for Christ coming to the Jews. Also we have this verse to support it: A light to lighten the Gentiles, and the glory of thy people Israel. (Lk 2:32) Christ was of "the seed of Abraham", of the tribe of Judah, and of the line of David. Gentile Christians seem to forget that Jesus of Nazareth was a Hebrew of the Hebrews.

But I don't feel less a part of God's people simply because I've been grafted in.

And that is as it should be. In the eyes of God there is neither Jew nor Gentile in the Body of Christ. The tragedy is that the Church is primarily Gentile, when it should be primarily Jewish. But that is entirely the fault of unbelieving Jews. Paul had to eventually turn away from the Jews altogether, and focus totally on the Gentiles. But Jesus was "made under the Law" meaning that He fully obeyed the Law of Moses in the manner that it should be obeyed. And then when He died, He declared "IT IS FINISHED". Of course that saying covers more than then end of the Law, and is a topic in itself.

Romans 3:25 KJV
Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God;

we can restructure this statement without altering its message, in order to emphasize the past, present, and/or future application.
for example:
Through the forbearance of God, God hath set forth Jesus to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past (the propitiation through faith is applicable all the way back to Adam).

Paul says something similar to this in Acts 17:30, "Although God overlooked the ignorance of earlier times, He now commands all people everywhere to repent. "

because...

Romans 3: 21, "But now, apart from the law, the righteousness of God has been revealed, as attested by the Law and the Prophets."

Sins can only be forgiven with repentance - this presumption that Jesus 'Lamb' died for the sins is contrary to Jesus teachings.......

The Father's justice demands that only a few be saved. If He saves all, there is no justice because we are all condemned already and sin would have to be allowed in heaven which means that He would no longer be sovereign. If He condemns all, He would be merciless. To show His mercy, He saves a few by baptizing them in His name by bestowing on them the state of being of repentance for their sins so that the sinner turns to Him for forgiveness.The Father in turn uses different means to bring him the knowledge of forgiveness of sins through Christ. Christ then baptizes the repentant sinner in His name by bestowing on him the faith needed to believe in His work on the cross for the forgiveness of his sins and in the name of the Holy Spirit when the cleansed but still sinful sinner gets perfected. He gets filled with the Spiritual essence of the Father and the Son ( the Holy Spirit) who fill his heart with the divine love necessary to obey the Spirit of the Law given by Christ which is to love God first and everyone else as he loves himself. You will not sin when all your thoughts and actions are motivated by Love for God and people. It is a state of being that cannot be self-induced. THe Father grants repentance, the Son erases, and the Holy Spirit perfects. Salvation is a state of being, not just a mental thing. God expects perfection which we cannot achieve on our own. Thank you Father, Son, and Holy Spirit for making it happen. All three are involved in our salvation. That is how you get baptized in the name of the Father, and of the Son, and of the Holy Spirit.

Romans 6:1-2,12-13 KJV
What shall we say then? Shall we continue in sin, that grace may abound?

Romans 6:2-6,12-13,15,17,21 KJV
God forbid. How shall we, that are dead to sin, live any longer therein? [3] Know ye not, that so many of us as were baptized into Jesus Christ were baptized into his death? [4] Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life. [5] For if we have been planted together in the likeness of his death, we shall be also in the likeness of his resurrection: [6] Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin. [12] Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof. [13] Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God. [15] What then? shall we sin, because we are not under the law, but under grace? God forbid. [17] But God be thanked, that ye were the servants of sin, but ye have obeyed from the heart that form of doctrine which was delivered you. [21] What fruit had ye then in those things whereof ye are now ashamed? for the end of those things is death.

Do you believe the sheep from another flock weren't always in God's plan? Or that the church because it is predominantly gentile has lost much of the Jewish influence and culture?

The sheep from another flock were always in God's plan of salvation. But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world. But I say, Did not Israel know? First Moses saith, I will provoke you to jealousy by them that are no people, and by a foolish nation I will anger you. (Rom 10:18,19) Paul was quoting Deuteronomy 32:21. In the eyes of Jews Gentiles were "no people" but now the Jews are being provoked to jealousy because the Church is predominantly Gentile. And the Church did lose much of Jewish influence, partly because Christians were persecuted by unbelieving Jews, and partly because rabbinic Judaism became more and more divorced from the Bible. The Talmud replaced the Tanakh for all practical purposes.

The Father's justice demands that only a few be saved. If He saves all, there is no justice because we are all condemned already and sin would have to be allowed in heaven which means that He would no longer be sovereign. If He condemns all, He would be merciless. To show His mercy, He saves a few by baptizing them in His name by bestowing on them the state of being of repentance for their sins so that the sinner turns to Him for forgiveness.The Father in turn uses different means to bring him the knowledge of forgiveness of sins through Christ. Christ then baptizes the repentant sinner in His name by bestowing on him the faith needed to believe in His work on the cross for the forgiveness of his sins and in the name of the Holy Spirit when the cleansed but still sinful sinner gets perfected. He gets filled with the Spiritual essence of the Father and the Son ( the Holy Spirit) who fill his heart with the divine love necessary to obey the Spirit of the Law given by Christ which is to love God first and everyone else as he loves himself. You will not sin when all your thoughts and actions are motivated by Love for God and people. It is a state of being that cannot be self-induced. THe Father grants repentance, the Son erases, and the Holy Spirit perfects. Salvation is a state of being, not just a mental thing. God expects perfection which we cannot achieve on our own. Thank you Father, Son, and Holy Spirit for making it happen. All three are involved in our salvation. That is how you get baptized in the name of the Father, and of the Son, and of the Holy Spirit.