How can we tell whether or not God’s Word is the True Gospel? Paul gives us three tests. First of all, the source test. The Gospel is not the invention of man. It is the Truth that God gave. God’s Word says, “But I [Paul] certify you, brethren, that the gospel which was preached of me is not after man. For I neither received it of man, neither was I taught it, but by the revelation of Jesus Christ” (Galatians 1:11-12). The Gospel is direct and divine revelation.
Secondly, the salvation test. The Gospel is authenticated by its message of salvation. You can’t beg, buy, borrow, or steal the salvation that comes from the Gospel of Jesus Christ. You can’t even earn it (see Ephesians 2:8-9). It was bought by the supernatural work of God through Christ on the cross.
The third test, is the subject test. First Corinthians 15:1-4 clearly tells us:
Moreover, brethren, I [Paul] declare unto you the gospel which I preached unto you, which also ye have received, and wherein ye stand; By which also ye are saved, if ye keep in memory what I preached unto you, unless ye have believed in vain. For I delivered unto you first of all that which I also received, how that Christ died for our sins according to the scriptures; And that He was buried, and that He rose again the third day according to the scriptures.
It is Jesus and Him alone who is the subject of the Gospel. He is the beginning, the end, and the forever of the Gospel.
LWF.org