The "forever" circumcision covenant was fulfilled/finished/completed by Jesus Christ's perfect sacrificial death on the cross. Please see Romans 5:10, Hebrews 5:8-10, and Hebrews 10:14 to see that Jesus Christ's bloody sacrificial death on the cross is the perfect blood sacrifice and that it is He alone who reconciled God to mankind.
This "biological descendant" circumcision covenant began with Abraham and it ended with Jesus Christ, the promised "offspring", who has united all of Abraham's children of faith (both Jew/Israelite and Greek/Gentile) through the circumcision of their hearts instead of through the circumcision of their flesh.
“Forever” does not always mean "never-ending" and I have put two examples of this fact at the end of this post.
"Genesis 17:3-5
When Abram was ninety-nine years old the Lord appeared to Abram, and said to him, “I am God Almighty; walk before me, and be blameless. 2 And I will make my covenant between me and you, and will multiply you exceedingly.” 3 Then Abram fell on his face; and God said to him, 4 “Behold, my covenant is with you, and you shall be the father of a multitude of nations. 5 No longer shall your name be Abram, but your name shall be Abraham; for I have made you the father of a multitude of nations. 6 I will make you exceedingly fruitful; and I will make nations of you, and kings shall come forth from you."
Abraham received this promise from God that he would become the father of many nations because of his faith and this promise was made before the circumcision covenant was made between Abraham and God and so the circumcision covenant applied only to his biological descendants. It is now known that this "forever" circumcision covenant was to last until the promised offspring's (Jesus Christ) death and resurrection. This offspring’s (Jesus Christ) death and resurrection did unite all of God's people (both Jew and Gentile) under one new covenant and the sign of this new covenant is the circumcision of the hearts of these peoples instead of the circumcision of the flesh of these peoples. Jesus Christ's new covenant is never-ending because Jesus Christ is both God and Man and so therefore He is called the one Mediator between God the Father and mankind. Jesus Christ received His Mediator status through the merits of His own perfect bloody sacrificial death and His resurrection from the dead. Please see 1 Timothy 2:3-6.
"Galatians 3:6-9
Thus Abraham “believed God, and it was reckoned to him as righteousness.” 7 So you see that it is men of faith who are the sons of Abraham. 8 And the scripture, foreseeing that God would justify the Gentiles by faith, preached the gospel beforehand to Abraham, saying, “In you shall all the nations be blessed.” 9 So then, those who are men of faith are blessed with Abraham who had faith."
"Galatians 3:15-18
15 To give a human example, brethren: no one annuls even a man’s will, or adds to it, once it has been ratified. 16 Now the promises were made to Abraham and to his offspring. It does not say, “And to offsprings,” referring to many; but, referring to one, “And to your offspring,” which is Christ. 17 This is what I mean: the law, which came four hundred and thirty years afterward, does not annul a covenant previously ratified by God, so as to make the promise void. 18 For if the inheritance is by the law, it is no longer by promise; but God gave it to Abraham by a promise."
"Galatians 3:23-29
23 Now before faith came, we were confined under the law (of Moses), kept under restraint until faith should be revealed. 24 So that the law was our custodian until Christ came, that we might be justified by faith. 25 But now that faith has come, we are no longer under a custodian; 26 for in Christ Jesus you are all sons of God, through faith. For as many of you as were baptized into Christ have put on Christ. 28 There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. 29 And if you are Christ’s, then you are Abraham’s offspring, heirs according to promise."
Abraham believed God's promise that he was to become the father of many nations. One of these many nations was to be kept separate from the other nations because the Messiah was to come from Abraham's biological descendants. God desired to have a relationship with this nation and to bless this nation with the knowledge and promise that the world's Savior was to be born from this nation. This nation was to be specifically blessed until the Savior was born. When the Savior was born, He was to unite all of Abraham's children, the peoples of faith in the Jewish nation and the peoples of faith in the Gentile nations, into just one nation whose father is Abraham.
The sign of this biological nation was that the males were to be circumcised. Jesus Christ is this promised offspring/Messiah/Savior and therefore His circumcision fulfilled/finished/completed the "forever" circumcision covenant and so circumcision is no longer helpful for God's people.
After the Savior Jesus Christ's death and resurrection from the dead, God’s people are now the people who believe in and obey God’s only begotten Son, Jesus Christ. See Romans 9:24-26
"Ephesians 2:11-16
Therefore remember that at one time you Gentiles in the flesh, called the uncircumcision by what is called the circumcision, which is made in the flesh by hands— 12 remember that you were at that time separated from Christ, alienated from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. 13 But now in Christ Jesus you who once were far off have been brought near in the blood of Christ. 14 For he is our peace, who has made us both one, and has broken down the dividing wall of hostility, 15 by abolishing in his flesh the law of commandments and ordinances, that he might create in himself one new man in place of the two, so making peace, 16 and might reconcile us both to God in one body through the cross, thereby bringing the hostility to an end."
The "dividing wall of hostility" is a metaphor taken from the wall that divided the court of the Gentiles from the court of the Israelites in the temple.
The circumcision covenant was made “after” God's promise to Abraham about his offspring Jesus Christ. Therefore the "forever" circumcision of the flesh covenant was made obsolete/no longer necessary by the death of Jesus Christ on the cross. The Law of Moses was also made obsolete by the death of Jesus Christ on the cross. The Law of Moses (the law of commandments and ordinances) did not begin until four hundred and thirty years after God's promise was made to Abraham and so this Law of Moses was also made obsolete by the death of Jesus Christ on the cross.
"Hebrews 8:13
In speaking of a new covenant he treats the first as obsolete. And what is becoming obsolete and growing old is ready to vanish away."
The Galatians were admonished for their mistaken belief that circumcision of the flesh was necessary under Jesus Christ's new covenant. The Galatians are told that those who become circumcised will sever themselves from Christ's grace of salvation.
"Galatians 3:1-4
O foolish Galatians! Who has bewitched you, before whose eyes Jesus Christ was publicly portrayed as crucified? 2 Let me ask you only this: Did you receive the Spirit by works of the law, or by hearing with faith? 3 Are you so foolish? Having begun with the Spirit, are you now ending with the flesh (circumcision)? "
"Galatians 5:3-5
I testify again to every man who receives circumcision that he is bound to keep the whole law. 4 You are severed from Christ, you who would be justified by the law; you have fallen away from grace. 5 For through the Spirit, by faith, we wait for the hope of righteousness."
The following are examples of other understandings for the word "forever" and they do not mean "never-ending".
1 Samuel 1:22
But Hannah went not up; for she said unto her husband, “I will not go up until the child (Samuel) is weaned, and then I will bring him, that he may appear before the Lord and there abide for ever.”
We do know that Samuel is not abiding there presently (he is deceased).
1 Kings 9:1-3
When Solomon had finished building the house of the Lord and the king’s house and all that Solomon desired to build, 2 the Lord appeared to Solomon a second time, as he had appeared to him at Gibeon. 3 The Lord said to him, ‘I have heard your prayer and your plea, which you made before me; I have consecrated this house that you have built, and put my name there for ever; my eyes and my heart will be there for all time.
Solomon's Temple (the house of the Lord) is no longer there in Jerusalem.
“Forever” (for ever) does not always mean “never-ending”.