The Apostle Paul was not ecumenical in the least. If a doctrine detracted from the sole work of Christ as redeemer and Saviour, he confronted and demolished it in short order in his articulate manner. To Paul, "anything goes" was a lie. There is no such thing as relative truth.
So what is the gospel? In Jesus' own beloved words, the gospel is the good news that Christ came to die for specific sinners, chosen of the Father, who will be housed in Heaven's blissful world for all of eternity.
This, in conjunction with New Covenant worship as opposed to worship under the Law Service, was the single most important issue in Paul's ministry. Without this truth, all others crumble. The Glory of Christ is at stake.
All that the Father giveth me shall come to me; and him that cometh to me I will in no wise cast out. For I came down from heaven, not to do mine own will, but the will of him that sent me. And this is the Father's will which hath sent me, that of all which he hath given me I should lose nothing, but should raise it up again at the last day. And this is the will of him that sent me, that every one which seeth the Son, and believeth on him, may have everlasting life: and I will raise him up at the last day. John 6:37-39
Then came the Jews round about him, and said unto him, How long dost thou make us to doubt? If thou be the Christ, tell us plainly. Jesus answered them, I told you, and ye believed not: the works that I do in my Father's name, they bear witness of me. But ye believe not, because ye are not of my sheep, as I said unto you. My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any [man] pluck them out of my hand. My Father, which gave [them] me, is greater than all; and no [man] is able to pluck [them] out of my Father's hand. I and [my] Father are one. John 10:24-30
These words spake Jesus, and lifted up his eyes to heaven, and said, Father, the hour is come; glorify thy Son, that thy Son also may glorify thee: As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him. And this is life eternal, that they might know thee the only true God, and Jesus Christ, whom thou hast sent. John 17:1-3
When Jesus therefore had received the vinegar, he said, It is finished: and he bowed his head, and gave up the ghost. John 19:30