The teaching of Scripture on circumcision is very clear in the Old Testament and not always so clear in the New Testament, mostly because of what Paul seems to say. Paul was a Torah teacher, a Torah lawyer it seemed, extremely skilled in knowledge of the Law of YHVH as well as in the human tradition/religion of Judaism. Peter states that Paul said some things which those who were unlearned in Torah (Gentiles), or unstable in faith (Jews) twisted and perverted. Paul never said he came to start yet another religion. He urged his readers to follow him as He followed Yeshua. If he followed Yeshua, then he followed the One who walked perfectly in the commandments, not twisting them. Paul bases his teaching on the Torah. That is the best place to begin learning about circumcision.
And God said to Abraham, "As for you, you shall keep my covenant, you and your offspring after you throughout their generations. This is my covenant, which you shall keep, between me and you and your offspring after you: Every male among you shall be circumcised. You shall be circumcised in the flesh of your foreskins, and it shall be a sign of the covenant between me and you. He who is eight days old among you shall be circumcised. Every male throughout your generations, whether born in your house or bought with your money from any foreigner who is not of your offspring, both he who is born in your house and he who is bought with your money, shall surely be circumcised. So shall my covenant be in your flesh an everlasting covenant. Any uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant." Genesis 17:9-14
Note that this is a covenant between Abraham, who was not a ‘Jew’ or even an ‘Israelite’, and YHVH. Remember, this predates the Torah being given at Mt Sinai/Horeb. It shows us some of the many elements of the Torah that were already there before it was formally given to the Israelites as a whole.
Note that in this passage in Genesis; it is only those born into Abraham’s family that belonged in the covenant, as well as those foreigners who were ‘bought with his money’ as servants, and those born to him from those servants.
And the LORD said to Moses and Aaron, "This is the statute of the Passover: no foreigner shall eat of it, but every slave that is bought for money may eat of it after you have circumcised him. No foreigner or hired servant may eat of it. It shall be eaten in one house; you shall not take any of the flesh outside the house, and you shall not break any of its bones. All the congregation of Israel shall keep it. If a stranger shall sojourn with you and would keep the Passover to the LORD, let all his males be circumcised. Then he may come near and keep it; he shall be as a native of the land. But no uncircumcised person shall eat of it. There shall be one law for the native and for the stranger who sojourns among you." Exodus 12:44-49
Here we find provision made for the foreigner (stranger) who lived among the native born Israelites as one of them. He was not to be treated differently. He didn’t have to observe the Passover, in order to live among the Israelites. But he was not called one of God’s people then either. IF he wanted to honour YHVH in observing the Passover, THEN he had to first become as one of them. ONE LAW for all God’s people.
Deuteronomy 10 and 11 speak about the faith that must be present and precede ritual observance. Note all the times the people are warned to keep all the commandments, and for how long they are to observe them all…
"And now, Israel, what does the LORD your God require of you, but to fear the LORD your God, to walk in all his ways, to love him, to serve the LORD your God with all your heart and with all your soul, and to keep the commandments and statutes of the LORD, which I am commanding you today for your good? Behold, to the LORD your God belong heaven and the heaven of heavens, the earth with all that is in it. Yet the LORD set his heart in love on your fathers and chose their offspring after them, you above all peoples, as you are this day. Circumcise therefore the foreskin of your heart, and be no longer stubborn. For the LORD your God is God of gods and Lord of lords, the great, the mighty, and the awesome God, who is not partial and takes no bribe. He executes justice for the fatherless and the widow, and loves the sojourner, giving him food and clothing. Love the sojourner, therefore, for you were sojourners in the land of Egypt…"
You shall therefore love the LORD your God and keep his charge, his statutes, his rules, and his commandments always…
You shall therefore keep the whole commandment that I command you today, that you may be strong…
And if you will indeed obey my commandments that I command you today…"
You shall therefore lay up these words of mine in your heart and in your soul, and you shall bind them as a sign on your hand, and they shall be as frontlets between your eyes. You shall teach them to your children, talking of them when you are sitting in your house, and when you are walking by the way, and when you lie down, and when you rise. You shall write them on the doorposts of your house and on your gates…
For if you will be careful to do all this commandment that I command you to do, loving the LORD your God, walking in all his ways, and holding fast to him…
"See, I am setting before you today a blessing and a curse: the blessing, if you obey the commandments of the LORD your God, which I command you today, and the curse, if you do not obey the commandments of the LORD your God, but turn aside from the way that I am commanding you today…
…you shall be careful to do all the statutes and the rules that I am setting before you today.
Now note this in Deuteronomy 30:6
And the LORD your God will circumcise your heart and the heart of your offspring, so that you will love the LORD your God with all your heart and with all your soul, that you may live.
You can also read Jeremiah 4:4, 9:25 and John 7:22 (where Yeshua talks about the 8th day circumcision not being given by Moses, but from ‘the fathers’ or Abraham, Isaac and Jacob). There were two customs, the one from the fathers (which Yeshua referred to) and the one from Moses (or supposedly from Moses) which had added human custom. See Acts 15:1. Paul made a distinction.
Yet remember, Paul had Timothy circumcised. So we cannot say he was against circumcision. We could say he spoke strongly against the misuse or misinterpretation of circumcision. It was not a commitment made to serve human religious tradition. It was never meant to show acceptance by Israel of foreigners, but by YHVH of the foreigners. Paul says:
Do not present your members (parts of your body) to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members (parts of the body) to God as instruments for righteousness. For sin will have no dominion over you, since you are not under law but under grace.
What then? Are we to sin because we are not under law but under grace? By no means! Do you not know that if you present yourselves to anyone as obedient slaves, you are slaves of the one whom you obey, either of sin, which leads to death, or of obedience, which leads to righteousness? But thanks be to God, that you who were once slaves of sin have become obedient from the heart to the standard of teaching to which you were committed, and, having been set free from sin, have become slaves of righteousness. Romans 6:13-18