I have a question: Didn't Jesus fulfill the Old Testament when he came in the flesh?
When Jesus was born, the Mosaic law was still in effect. He was circumcised and presented to God as the first born male of Mary and Joseph as the old law prescribed: "And when eight days had passed, before His circumcision, His name was then called Jesus, the name given by the angel before He was conceived in the womb. And when the days for their purification according to the law of Moses were completed, they brought Him up to Jerusalem to present Him to the Lord (as it is written in the Law of the Lord, Every firstborn male that opens the womb shall be called holy to the Lord"), and to offer a sacrifice according to what was said in the Law of the Lord, "A pair of turtledoves or two young pigeons"" (Luke 2: 21-24). Later on, we see Jesus and his parents going to Jerusalem for the Passover: "Now His parents went to Jerusalem every year at the Feast of the Passover. And when He became twelve, they went up there according to the custom of the Feast; and as they were returning, after spending the full number of days, the boy Jesus stayed behind in Jerusalem. But His parents were unaware of it, but supposed Him to be in the caravan, and went a day's journey; and they began looking for Him among their relatives and acquaintances" (Luke 2:41-44).
Jesus did not come to destroy the Law, but to complete it when He was on earth. As Jesus stated: "Do not think that I came to abolish the Law or the Prophets; I did not come to abolish but to fulfill. For truly I say to you, until heaven and earth pass away, not the smallest letter or stroke shall pass from the Law until all is accomplished" (Matthew 5:17-18). Thus the old law was abolished when Jesus died on the cross for the remission of mankind's sins.
"For He Himself is our peace,who made both groups into one and broke down the barrier of the dividing wall,by abolishing in His flesh the enmity, which is the Law of commandments contained in ordinances, so that in Himself He might make the two into one new man, thus establishing peace" (Ephesians 2:14-15).
"Having canceled out the certificate of debt consisting of decrees against us, which was hostile to us; and He has taken it out of the way, having nailed it to the cross" (Colossians 2:14).