Perhaps I’m taking the word “gospel” in too narrow a sense. I’m thinking of it as expressed in the verses I referenced:
John 3:16
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
1 Cor. 15:1-4
Moreover, brethren, I 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:
1 John 4:9-10
In this was manifested the love of God toward us, because that God sent his only begotten Son into the world, that we might live through him. Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins.
It’s always a good idea to infer definitions from the context of scripture. I always believed the word gospel to mean “the teachings of Jesus and his apostles” and “the good news.”
2 Corinthians 11:3-4, 3 But I fear lest, as the serpent seduced Eve by his subtilty, so your minds should be corrupted, and fall from the simplicity that is in Christ. 4 For if he that cometh preacheth another Christ, whom we have not preached; or if you receive another Spirit, whom you have not received; or another gospel which you have not received; you might well bear with him.
John 20:28-29, 28 Thomas answered, and said to him: My Lord, and my God. 29 Jesus saith to him: Because thou hast seen me, Thomas, thou hast believed: blessed are they that have not seen, and have believed.
The Gospel of John starts off stating: “the Word was with God, and the Word was God.” Later on in John’s gospel the doubting Thomas doesn’t believe his brothers when they said that they had seen the risen Lord Jesus. When Jesus appeared and he felt his wounds from the cross, Thomas proclaimed Jesus to be “his Lord and his God,” which literally means
the Lord of me and the God of me in greek. Based on the context of this verse, we can conclude that Thomas already knew that Jesus was his Lord and his God. He merely professed this already understood truth once he realized that Jesus had indeed risen from the dead and was standing in front of him.
In 2 Corinthians 11 we see Paul echoing very similar statements to those mentioned in Galatians 1:6-9, but he does go a little further this time. He does warn us of people coming preaching “a different Christ” along with those that will preach a different Gospel.
There are several groups of “Christians” that believe in an apostasy having occurred, and who don’t believe in the nature of the Godhead in exactly the same way as the Orthodox (or as the Roman Catholics, who deviate slightly from Orthodoxy themselves.)
You are correct, and Catholics believe in this very apostasy you are referring to. Satan deceives us, that is just what he does. Of course he is going to pull people away from the truth in any way he can, whenever he can (Acts 20:28.)
Yet who confidently proclaim that Jesus Christ is the only begotten Son of God; who died for our sins, was buried, and rose again; and is the one we must believe in to gain eternal life. Mormons also believe these things.
Tieing this in with 1 Corinthians 15:1-4, what Paul and you are stating is the overall message of the Gospel. Is this the entirety of the Gospel that was taught from the time of Christ and on, no it is not.