But I make known to you, brethren, that the gospel which was preached by me is not according to man. For I neither received it from man, nor was I taught it, but it came through the revelation of Jesus Christ.
Galatians 1:11 (NKJV)
The Apostle Paul told the Galatians that the gospel should not be understood as a human tradition but rather as God's revelation to all peoples. We cannot add to the gospel or even subtract from the gospel—but cling steadfastly to the good news of God's salvation through Jesus Christ.
By adding to the gospel, we actually reject grace as the basis for God's acceptance of us and demand that we earn our salvation through works. In reality, whenever we remove something from the gospel, we empty it of its power to sanctify saints and save sinners.
Sometimes, believers assume there are many versions of the gospel, and they are free to select and follow the version they prefer. Despite the fact that four of the gospels are focused on the historical Jesus, only one gospel asserts that there is no other name under heaven by which we may be saved. (Acts 4:12) Nobody can therefore claim to have been saved by a "better" gospel than the rest of us. Furthermore, this "better" gospel is probably a distorted version of the genuine gospel.
Paul, for instance, received the gospel message directly from Jesus, and when Paul compared what he taught to what Peter and James were preaching, they were completely in agreement. We are therefore all justified and saved by the same gospel, despite the fact that we may come from various churches with various doctrinal emphasises.
How can you be certain that the gospel you follow is true?
You will start to recognise God's amazing presence and power in your life as you hear and believe the gospel in all of its fullness. As you get to know God the Father and Jesus Christ the Lord better and more personally, you'll notice an ever-increasing grace and peace in your life (2 Peter 1:2).
Don't think of the gospel as a requirement to simply be saved or as a bible lesson for new and young believers. You need the gospel more than you think you do. You need the gospel every day and in every stage of your life.