Assuming your question is not simply rhetorical, it was the gospel or good news of Christ’s kingdom, such good news that it was to be preached in all the earth before the end.
Was it the same as that preached to Abraham? Indeed it was!
Abraham was told that by means of his “seed” all the nations of earth would be blessed and that was good news since it revealed more of what that prophecy in Eden meant.
Is that really ‘All you want to know’?
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The Roman Catholic Church has reached a significant portion of “All the nations of the earth.” There aren't many other churches I know of that have. When I see a church that has a rich congregation and a rich pastor but no missionaries, I know they are, at least in my mind, false.
The Roman Church has spent a lot of gold and and blood proclaiming The Gospel. The Mormons for their small size of about 150th the size of the Roman Church have a remarkable number of missionaries. Who else is proclaiming the Gospel like the Catholics and Mormons. What is the motivation of the Catholics and Mormons? It certainly isn't earthly gain. Catholic pastors and leadership don't get rich from their efforts and Mormons don't get anything, they pay their own way.