This is a correct understanding of my initial question. Israel was commanded to keep the Passover and Sabbath "forever" which, if "forever" means for all time, would preclude abrogation in the New Testament era, and would mean that to set them aside would indeed be sin. That does not mean that these things were a means of salvation, any more that obeying the speed limit is a means of salvation. That was the original question, But this question leads to a broader question which Matthew has correctly raised...why were Jewish Christians in the New Testament still keeping the Law?