"I am astonished that you are so quickly deserting the One who called you to live in the grace of Christ and are turning to a different gospel--which is really no gospel at all. Evidently some people are throwing you into confusion and are trying to pervert the gospel of Christ." (Galatians 1:6-7)Paul was shocked that these Galatians got off track so soon after their conversion. They chose rules over relationship with Christ. They turned to a different gospel which is indeed no gospel at all. Anything that adds to or subtracts from the gospel of God's grace/the gospel of Jesus Christ is false. Sinners are saved by grace alone and justified by faith in Christ alone. False teachers in Galatia (the Judaizers) taught that one was justified by works of the Mosaic law (such as the necessity of circumcision). We must be so familiar with the truth (God's Word) that we may detect any false teaching/false gospel immediately and avoid being confused or deceived. Be careful; watch out for those who twist the Scriptures and change the truth concerning Christ.
"Heresy is not so much the total denial of the truth as a perversion of it." (A.W. Pink)