Let me suggest two approaches that I think give us insight into what the early Church thought about the subject. The first is Paul's point in First Corinthians chapter 15. Paul reminds the Corinthians of the Gospel, which is; "Christ died for our sins, according to the scriptures. He was buried. He rose again on the third day, according to the scriptures and he was seen by upwards of five hundred witnesses."
The Gospel is thus reduced to four key points: Christ died for our sins, he was buried, he arose from the dead, he was seen by witnesses. But notice, two of the statements have a modifier: DIED and AROSE are modified by the phrase, according to the scriptures. Why not the other two? How about this, Christ's death and resurrection were fulfillments of prophecy, while his burial and sightings by many witnesses are offered as tangible EVIDENCE that he indeed died and rose again.
I conclude that the Gospel message is quite simple; Christ died for our sins and rose from the dead.
Fine, that's the gospel message, sufficient for salvation, but what do you need to know to be fully informed about the foundation of the Christian life? Again, the early Church is already there for you. If you read the early Church Fathers--the leaders following the Apostles--you find many references to "The Apostles Doctrine." This is the message or KEREGMA of the early church. But what is that message?
Simple, it is found in the messages in the Book of Acts. Read them. Acts 2:14-39, 3:11-26, 4:8-12, 5:29-32, 7:52-56, 9:19-20, 10:34-43, 13:22-41, 16:31, 17:3, & 18-31, 18:5 & 28, 22:14-16 and 26:23 & 29.
In those messages the Apostles taught six concepts, over and over. Not every message has all six but the six are repeated in the ongoing teaching of the Apostolic Church. They are:
1. Jesus came from God
2. You and I killed Him
3. God raised Him up
4. He sends the Holy Spirit
5. Believe and Repent
6. He’s coming back
I say with confidence that THIS is the bedrock of Christian teaching. In fact it ought to be the message we preach today. Instead of worrying about which Church is right, or which baptism practice will get you into heaven or are you working hard enough (you're not and never can so trust God's grace), let's proclaim the Gospel. Jesus Christ is God become human flesh. He died for our sins, conquered death on our behalf and is coming back. THAT my friend is a call to believe and repent.
What a concept.