The Apostle Paul lamented that even in his day, some were distorting the gospel of Christ (Galatians 1:7).
Some brethren at Corinth denied that there is such a thing as resurrection from the dead. If that were true, then the basic facts of the gospel—that Jesus died for our sins, was buried, then rose the third day—would be false. Where does that leave one? “And if Christ has not been raised, your faith is worthless; you are still in your sins” (1 Corinthians 15:17).
Some brethren in Asia denied that Christ came in the flesh, that deity took on human form (1 John 4:2-3; 2 John 7). If that were true, Jesus was not who He said He was. Where does that leave one? “Anyone who goes too far and does not abide in the teaching of Christ, does not have God” (2 John 9a).
Some brethren in Galatia said that salvation in Christ requires keeping the Law of Moses (at least, parts of it) as well. Where does that leave one? “You have been severed from Christ, you who are seeking to be justified by law; you are fallen from grace” (Galatians 5:4).
Distorting the gospel—by denying its facts, by altering Jesus’ identity, or by tampering with its requirements— is disastrous. No wonder Paul pronounced a curse on anyone who does so (Galatians 1:8-9).