I think that that should be understood in context.Paul would preach to the gentiles that salvation is by faith, not by the law.
Many Jews rejected Jesus' authority, and emphasised "the law".
Could there be salvation without belief in Jesus' authority? No.
..not good..Luther even added a word to a passage in the Pauline writings to read 'only' by faith.
People will believe what they want to believe. They can decide for themselves...you should not be telling Christians what to believe IMO