What was the reasoning behind circumcision?
God made a Covenant with Abraham. In the Covenant God made promises to Abraham and his descendants. In these promises we find the promise of resurrection to eternal life and an eternal possession on the earth through a descendant of Abraham in whom all nations would be blessed.
Genesis 12:1 Now the LORD had said unto Abram, Get thee out of thy country, and from thy kindred, and from thy father’s house, unto a land that I will shew thee: 2 And I will make of thee a great nation, and I will bless thee, and make thy name great; and thou shalt be a blessing: 3 And I will bless them that bless thee, and curse him that curseth thee: and in thee shall all families of the earth be blessed.
Genesis 12:14 And the LORD said unto Abram, after that Lot was separated from him, Lift up now thine eyes, and look from the place where thou art northward, and southward, and eastward, and westward: 15 For all the land which thou seest, to thee will I give it, and to thy seed for ever. 16 And I will make thy seed as the dust of the earth: so that if a man can number the dust of the earth, then shall thy seed also be numbered. 17 Arise, walk through the land in the length of it and in the breadth of it; for I will give it unto thee.
Genesis 17:1-4, 7 And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect. 2 And I will make my covenant between me and thee, and will multiply thee exceedingly. 3 And Abram fell on his face: and God talked with him, saying, 4 As for me, behold, my covenant is with thee, and thou shalt be a father of many nations. 5 Neither shall thy name any more be called Abram, but thy name shall be Abraham; for a father of many nations have I made thee. 6 And I will make thee exceeding fruitful, and I will make nations of thee, and kings shall come out of thee.... 7 And I will establish my covenant between me and thee and thy seed after thee in their generations for an everlasting covenant, to be a God unto thee, and to thy seed after thee. 8 And I will give unto thee, and to thy seed after thee, the land wherein thou art a stranger, all the land of Canaan, for an everlasting possession; and I will be their God. 9 And God said unto Abraham, Thou shalt keep my covenant therefore, thou, and thy seed after thee in their generations.
The visible sign of this Covenant was to be circumcision.
Genesis 17:10-14 This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised. 11 And ye shall circumcise the flesh of your foreskin; and it shall be a token of the covenant betwixt me and you. 12 And he that is eight days old shall be circumcised among you, every man child in your generations, he that is born in the house, or bought with money of any stranger, which is not of thy seed. 13 He that is born in thy house, and he that is bought with thy money, must needs be circumcised: and my covenant shall be in your flesh for an everlasting covenant. 14 And the uncircumcised man child whose flesh of his foreskin is not circumcised, that soul shall be cut off from his people; he hath broken my covenant.
Genesis 17:24 And Abraham was ninety years old and nine, when he was circumcised in the flesh of his foreskin. 25 And Ishmael his son was thirteen years old, when he was circumcised in the flesh of his foreskin. 26 In the selfsame day was Abraham circumcised, and Ishmael his son. 27 And all the men of his house, born in the house, and bought with money of the stranger, were circumcised with him.
When Isaac was born, he was circumcised as a token of the covenant God made with Abraham.
Genesis 21:4 And Abraham circumcised his son Isaac being eight days old, as God had commanded him.
Circumcision passed through to Israel, Abraham’s descendants, and was re-affirmed in the Old Covenant which God made with Israel at Sinai
Leviticus 12:1-3 And the LORD spake unto Moses, saying, 2 Speak unto the children of Israel, saying, If a woman have conceived seed, and born a man child: then she shall be unclean seven days; according to the days of the separation for her infirmity shall she be unclean. 3 And in the eighth day the flesh of his foreskin shall be circumcised.
Physical circumcision was intended to represent the ‘circumcision’ of the heart. A desire to serve God rather than our own fleshly lusts. Israel failed to keep the Covenant and God promised a New Covenant.
Jeremiah 31:31-34 Behold, the days come, saith the LORD, that I will make a new covenant with the house of Israel, and with the house of Judah: 32 Not according to the covenant that I made with their fathers in the day that I took them by the hand to bring them out of the land of Egypt; which my covenant they brake, although I was an husband unto them, saith the LORD: 33 But this shall be the covenant that I will make with the house of Israel; After those days, saith the LORD, I will put my law in their inward parts, and write it in their hearts; and will be their God, and they shall be my people. 34 And they shall teach no more every man his neighbour, and every man his brother, saying, Know the LORD: for they shall all know me, from the least of them unto the greatest of them, saith the LORD: for I will forgive their iniquity, and I will remember their sin no more.
The writer to the Hebrews explains:
Hebrews 8:8-12 For finding fault with them, he saith, Behold, the days come, saith the Lord, when I will make a new covenant with the house of Israel and with the house of Judah: 9 Not according to the covenant that I made with their fathers in the day when I took them by the hand to lead them out of the land of Egypt; because they continued not in my covenant, and I regarded them not, saith the Lord. 10 For this is the covenant that I will make with the house of Israel after those days, saith the Lord; I will put my laws into their mind, and write them in their hearts: and I will be to them a God, and they shall be to me a people: 11 And they shall not teach every man his neighbour, and every man his brother, saying, Know the Lord: for all shall know me, from the least to the greatest. 12 For I will be merciful to their unrighteousness, and their sins and their iniquities will I remember no more.
The New Covenant replaces the Old.
Hebrews 8:13 In that he saith, A new covenant, he hath made the first old. Now that which decayeth and waxeth old is ready to vanish away.
Jesus Christ is the mediator of the New Covenant
Hebrews 12:24 And to Jesus the mediator of the new covenant, and to the blood of sprinkling, that speaketh better things than that of Abel.
To have a part in the New Covenant, we must be baptised into Jesus Christ by immersion in water. Physical circumcision is not required, but circumcision of the heart is.
Romans 2:28 For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh: 29 But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God.
Romans 3:30 Seeing it is one God, which shall justify the circumcision by faith, and uncircumcision through faith.
1 Corinthians 7:19 Circumcision is nothing, and uncircumcision is nothing, but the keeping of the commandments of God.
Colossians 2:11 In whom also ye are circumcised with the circumcision made without hands, in putting off the body of the sins of the flesh by the circumcision of Christ:
Colossians 3:11 Where there is neither Greek nor Jew, circumcision nor uncircumcision, Barbarian, Scythian, bond nor free: but Christ is all, and in all.
I hope you find this helpful.