What is the gospel?2 Thess 1:7-9 He will punish those who do not know God and do not obey the gospel of our Lord Jesus. 9 They will be punished with everlasting destruction and shut out from the presence of the Lord and from the majesty of his power.What I taught and what is still taught by many congregations is that the word gospel in this verse includes everything in the NT.
All the Christians on the day of Pentecost obeyed the gospel before any of the New Testament was written and all the Christians who died at the orders of Saul died before any of the New Testament was written.
In about 58 a.d. Paul wrote Galatians, the second book of the New Testament to be written. All the Galatians had obeyed the gospel and some wanted to leave it, before they read Galatians, and probably before they read any book of the New Testament. Galatians may be part of the gospel to some Christians, but it certainly was not part of the gospel to the Galatians.
The NT Christians did not need Galatians, Romans, Acts, Matthew or other letters to be in fellowship with God. Faith in the death, burial, resurrection, ascension and the promises through that sacrifice placed them in fellowship with God.
Why were the epistles of Paul, Galatians, Romans, Corinthians, etc. written? It appears that some of the churches wrote to Paul with questions. They wanted answers to some of the things they didn't understand. Paul heard that some churches were having problems and Paul wrote these churches to correct some problems. Have you ever noticed that we would have a very small New Testament if the church had been perfect? Most of the New Testament was written to correct some problem in the church. Some do apply to us and some don't apply, but we have a multitude of principles that we can use.
In other words, "the Gospel" does not equate to "the New Testament".
Some might respond that the early Christians had all the information provided by the New Testament via supernatural revelation, even if it had not yet been written down. But in at least the question of whether Gentile Christians had to submit to the Jewish Law (Acts 15), there were Gentile Christians who had obeyed "the Gospel" prior to this matter being settled. Thus, in this issue at least, the New Testament contains information which those Gentile (or Jewish) Christians could not have known even though they had obeyed "the Gospel".
Simply put, once again, "the Gospel" is not the same as "the New Testament".