The Jewish believers in New Testament times had grown up with the Law of Moses. It was the structure through which they had learned to worship God, and the framework for their relationship with God. To have that suddenly removed would have been very difficult for them to understand as they needed to learn to worship God in new ways.
Everyone is attached to the religion they were taught as children, and it is always hard to change even when you know that change is necessary and your new understanding is correct.
The book of Galatians was written to correct the false teaching that Christians needed to keep the law of Moses. It is full of instruction on this issue. Here are some examples.
Galatians 2:15-16
We ourselves are Jews by birth and not Gentile sinners; 16 yet we know that a person is not justified by works of the law but through faith in Jesus Christ, so we also have believed in Christ Jesus, in order to be justified by faith in Christ and not by works of the law, because by works of the law no one will be justified.
Galatians 3:24-26
So then, the law was our guardian until Christ came, in order that we might be justified by faith. 25 But now that faith has come, we are no longer under a guardian, 26 for in Christ Jesus you are all sons of God, through faith.
Galatians 4:9-11
But now that you have come to know God, or rather to be known by God, how can you turn back again to the weak and worthless elementary principles of the world, whose slaves you want to be once more? 10 You observe days and months and seasons and years! 11 I am afraid I may have labored over you in vain.
Galatians 5:1-4
For freedom Christ has set us free; stand firm therefore, and do not submit again to a yoke of slavery. 2 Look: I, Paul, say to you that if you accept circumcision, Christ will be of no advantage to you. 3 I testify again to every man who accepts circumcision that he is obligated 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.