The heresy of legalism, sometimes called the Galatian heresy, because Paul specifically refutes it in his letter to the Galatians, seems to be on the increase today. You might find it running rampant particularly among Hebrew-roots and Messianic Jewish organizations.
Basically this heresy means telling Christians that they must live like Jews in order to be saved and/or to please God (by putting themselves under the Law of Moses).
It does seem to me from reading Acts 21:17-26 and 1 Corinthians 7:17-20 that there is a case to be made for Jews who convert to Christ to continue practicing their customs according to the Law of Moses, just to be fair and clear. Still, understood from an overall reading of the Bible, this Jewish Mosaic observance would be in freedom and grace under Christ, and certainly not as a means of salvation. In other words, if you are a Jew and prefer, even after converting to faith in Christ, to continue keeping your customs, you are not sinning, as long as you realize those customs do not save you. Whether we are Jew or Gentile, faith in Christ is the only thing that can save.
The Sermon on the Mount
The sermon Jesus gives in Matthew 5 is a favorite for legalists, taken out of context, to justify their doctrine of adherence to the Law. As the article I will link below points out, one must understand the context in which this was given- Jesus was speaking to legalistic Jews at the time, under the covenant of Law- not grace- and was correcting their lenient “sweep sin under the rug” application of those laws. It would be incorrect to apply Jesus’ words in that sermon, where He says “Unless your righteousness exceeds that of the scribes and the pharisees, you will not enter the kingdom of heaven,” to modern-day Christians. Is Jesus teaching Sinless Perfectionism here? Works-based salvation? No! He is showing the Jews that their understanding of God’s standard is not strict enough. Only perfection is good enough for God, and they could not attain it- not even the pharisees could attain to it, and they were viewed as the holiest people on the planet. Only Jesus’ righteousness is good enough for God. Read more on that in this article:
The legalists do everything in their power to make it seem as if the Bible teaches the exact opposite of what it really does on this issue. The only solution is to read the scriptures for yourself. Here are the links to my scripture references:
** Please note that I have included some extra-biblical quotes from the Epistle of Barnabas, a very early Christian writing, in this list. They are clearly marked off with (ECW) next to the citations. This is by no means because I consider them authoritative in the same way as the Bible, but rather to show what the very earliest Christian leaders themselves believed. If there is a dispute among modern believers as to how to interpret certain Biblical passages, it can be immensely helpful to consult the people who lived at the same time as the apostles themselves and probably even conversed with them directly. Very often it is falsely-alleged by legalists that the earliest Christians prior to Roman Catholicism actually preached adherence to the Law of Moses. This could not be further from the truth.
Here is an in-depth article on the teaching of Paul the apostle on this subject:
P.S. In dealing with legalist heretics you may occasionally come upon the charge that Paul was a false apostle since he so clearly teaches freedom from the Law. In response of course we can quote 2 Peter 3:16 which shows without doubt that Peter the apostle fully accepted Paul and considered his work to be scripture. I find this article helpful:
I also highly recommend the playlist on YouTube by The Fuel Project called “Stay Free” for a thorough understanding of this issue, especially part 21 and some of the following videos in the playlist, which specifically deal with Judiazers.