Your "opponents" here are basically trying to fit what Paul revealed about the mystery of the grace dispensation, into everything else in the 4 gospels and the book of James.
For example, after they understood Paul, they go back to a verse like John 3:16 or John 6:29 and conclude, "See, Jesus ONLY said to believe in him will have eternal life, he did not say be water baptized, obey the Law, etc etc".
They ignore what Jesus said about keeping the Law elsewhere in the 4 gospels, and also reinterpret what James was trying to tell the Jews in his letter that faith without works is dead, from the perspective of Paul.