But some men came down from Judea and were teaching the brethren, "Unless you are circumcised according to the custom of Moses, you cannot be saved." Acts 15:1 (RSV)
C.S. Lewis describes it as 'Christianity and...', the habit of folk adding something to the faith as necessary to be a real Christian. The Church was still in its early days when some within decided that one had to follow the law of Moses before one could be a believer.
Salvation no longer was through the death and resurrection of Christ alone; it was 'Jesus and...'. Such is a severe error and danger to all whom the Gospel is proclaimed. Somehow the death and resurrection of Jesus aren't enough. God needs our help in completing salvation. It is the sin of Adam in a new dress. We desire to be God, and if we can't have that, we at least want to be partners with God in salvation.
Many things can damage the believer and hinder faith. Against these, the believer must be vigilant. We cannot add to what Jesus has done for us that will make salvation complete.
We are saved by grace through faith. It is not a work that we may boast. Nor should anyone lay an additional burden on those whom the Lord has called through faith. It is Jesus only by whom we must be saved. Thanks be to God that this is so.