Let's be careful not to assume different gospels from the one that was preached by the first apostles:
Speaking of switching doctrines to those of man that might keep us from the Kingdom (not heaven). Sorry but the first one preached was by Jesus and He even said it was why He was sent; to proclaim it and fulfil, the Gospel of the Kingdom. He and the 12 mention it around 72 times, and also it is said in scripture at least three times, Paul taught the Gospel of the Kingdom for years. The good news was God was coming back to live among man and dethrone man from governing and oppressing one another. That is salvation enough but Jesus died for us, yes. Only in that death would not be a finality, but now we would have the Kingdom as an option thanks to the grace of God. But of course man, being full of self interest, will either not want to give up control, or if getting into Christianity makes it about what's in it for us rather than about the will of God in His good news of the Kingdom.
Mark 1:14 Now after that John was put in prison, Jesus came into Galilee, preaching the
gospel of the kingdom of God,15 And saying, The time is fulfilled, and the
kingdom of God is at hand: repent ye, and believe the gospel.
Matthew 24: 11 And many false prophets shall rise, and shall deceive many. 12 And because iniquity shall abound, the love of many shall wax cold. 13 But he that shall endure unto the end, the same shall be saved. 14 And this GOSPEL OF THE KINGDOM shall be preached in all the world for a witness unto all nations; and then shall the end come.
Luke 4: 43 And he said unto them, I must preach the kingdom of God to other cities also: for therefore am I sent.
etc.
Acts 19:8 He (Paul) went into the synagogue, and spake boldly for the space of three months, disputing and persuading the things concerning the Kingdom of God
Acts 28: 23 And when they had appointed him a day, there came many to him into his lodging; to whom he expounded and testified the kingdom of God, persuading them concerning Jesus, both out of the law of Moses, and out of the prophets, from morning till evening. 24 And some believed the things which were spoken, and some believed not.
Acts 28: 30 And Paul dwelt two whole years in his own hired house, and received all that came in unto him, 31 Preaching the kingdom of God, and teaching those things which concern the Lord Jesus Christ, with all confidence, no man forbidding him.