Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you alway, even unto the end of the world. Amen.
I've written about this before here. You've got one verb and three participles. The verb, the point of the sentence, is "teach all nations," and "teach" is matheteusate, a plural aorist imperative, "make disciples." As a side note, but important, I believe that the plural pronoun "them" refers to the disciples made. Certainly one should see no problem with baptism as a part of discipleship, but those baptized are already disciples. However, even Matthew 28:19-20 doesn't tell us what the gospel is that is preached. That leaves us with Luke 24. I think we should look at vv. 44-48.
44 And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me. 45 Then opened he their understanding, that they might understand the scriptures, 46 And said unto them, Thus it is written, and thus it behoved Christ to suffer, and to rise from the dead the third day: 47 And that repentance and remission of sins should be preached in his name among all nations, beginning at Jerusalem. 48 And ye are witnesses of these things.
In v. 44, Jesus said these were the words that He had spoken while He was with them. He also says that this is the message of the Old Testament. What Jesus spoke was New Testament, but it wasn't anything different than what could be preached from the Old Testament. Jesus opened the disciples' understanding to the teaching about Himself in the Old Testament, because scripture is what they would preach, which included the suffering, death, and resurrection of Christ. The Christ would save them by His death, burial, and resurrection.
To carry on the work of the Lord, to preach the gospel, to make disciples -- all parallel ideas to this -- what were these men to preach? What was the gospel response? "Repentance and remission of sins should be preached in his name" (v. 46). This is the one place that says what to preach -- Repentance and remission of sins in his name. The one Great Commission passage that tells what to preach says preach repentance. This repentance would be like Jesus preached it all through the gospels, because Jesus said that repentance was included in the words which He spoke unto them. What Jesus preached was repentance.
There is no doubt that Jesus included faith or belief in this message to preached, but He doesn't actually say it in any of the Great Commission passages. The disciples we know heard, "Preach repentance." They also heard "remission of sins." Men needed remission of sins. Their sins were the problem. Do you think that they were thinking, when they heard that message, that they could just keep sinning? Of course not.
Another point. We shouldn't leave out "in his name." "His name" is what represents Jesus. It is Who He is. Part of preaching Jesus from the Old Testament makes sancrosanct certain attributes of Who Jesus is -- He's God, He's Lord, He's Savior. He's the King. He's going to rule the world. He's going to judge the world.
One last point. Part of the Great Commission was being witnesses of these things. What things? The things Jesus just talked about from what He said during His ministry and from the Old Testament. Jesus taught them from the Old Testament because that's how people are saved, by hearing the preaching of the Word of God. This is what He wanted them to talk about.
What I am hearing from the "free grace" men, from their position, is that if you preached what Jesus said to preach in His Great Commission, you would be adding works to grace or frontloading works. I think we're safe with sticking with what Jesus actually said to do. Let's do that.