What man has done in his vain nature was supplant the Gospel of Truth with "another gospel". Now those who believe in this "other gospel" in their minds they're not wrong. The Apostle Paul warned us about this "other gospel" as he so clearly defines when speaking to the Galatians:
6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
Paul is saying there is really only one Gospel but men began causing confusion by perverting it
8 But though we, or an angel from heaven, preach any other gospel unto you than that which we have preached unto you, let him be accursed.
9 As we said before, so say I now again, if any man preach any other gospel unto you than that ye have received, let him be accursed.
Paul mentions this twice
10 For do I now persuade men, or God? or do I seek to please men? for if I yet pleased men, I should not be the servant of Christ.
11 But I certify you, brethren, that the gospel which was preached of me is not after man.
In other words, he did not receive this teaching in a seminary
12 For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ.
As we all should receive through the Holy Spirit
So through Doctrines of men being perpetrated against those worshipping in Truth and Spirit carries on thereby convincing followers they are correct , "in another gospel"
Paul wrote the letter of Galatians to the believers in churches at Galatia - a province of Rome - that he, himself, founded. The epistle was written around 57-58 A.D. What I find interesting is that false doctrines were already starting to infiltrate the early church. From the following site:
The churches of Galatia were founded by Paul himself (Acts 16:6; Gal. 1:8; 4:13, 19). They seem to have been composed mainly of converts from heathenism (4:8), but partly also of Jewish converts, who probably, under the influence of Judaizing teachers, sought to incorporate the rites of Judaism with Christianity, and by their active zeal had succeeded in inducing the majority of the churches to adopt their views (1:6; 3:1). This epistle was written for the purpose of counteracting this Judaizing tendency, and of recalling the Galatians to the simplicity of the gospel, and at the same time also of vindicating Paul's claim to be a divinely-commissioned apostle.
The great question discussed is, Was the Jewish law binding on Christians? The epistle is designed to prove against the Jews that men are justified by faith without the works of the law of Moses.
The Epistle to the Galatians and that to the Romans taken together “form a complete proof that justification is not to be obtained meritoriously either by works of morality or by rites and ceremonies, though of divine appointment; but that it is a free gift, proceeding entirely from the mercy of God, to those who receive it by faith in Jesus our Lord.”
Just like the early church, we see that the exact same perversion of the gospel is still alive within the faith. Only by listening with our hearts to the Holy Spirit through the teachings he revealed in the Holy Bible can we know for sure what the real, true gospel is and it has never changed from being by grace through faith in Christ Jesus.