Under Covenant, Christ, never taught in a "Christian Church," there were no such thing. He taught in Jewish Synagogues and preached "the gospel of the kingdom" and is why he said "salvation is of the Jews."
Matthew 4:23 And Jesus went about all Galilee, teaching in their synagogues, and preaching the gospel of the kingdom, and healing all manner of sickness and all manner of disease among the people
(Also see 9:35, 24:14 & Mark 1:14)
John 4:22 Ye worship ye know not what: we know what we worship: for salvation is of the Jews.
Under Grace, Christ sent Paul, to teach the message for "the church, Which is his body." and he did NOT preach "the gospel of the kingdom." No, Paul preached "the gospel of the grace of God." aka "the gospel of Christ."
Acts 20:24 But none of these things move me, neither count I my life dear unto myself, so that I might finish my course with joy, and the ministry, which I have received of the Lord Jesus, to testify the gospel of the grace of God.
Romans 1:16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
As to Romans 1:16, do note that many bibles (other than the King James) leave off the words "of Christ." In doing so, they have caused many to err by failing to see the distinction between "the gospel of the kingdom" and "the gospel of Christ." By only saying "the gospel" it's assumed that it's referring to "the gospel of the kingdom."
The "gospel of the kingdom" is a gospel that had NO MENTION of God deciding to stop imputing/charging sins to humanities account. Something that was only later revealed in the "gospel of Christ" that the Lord gave to the apostle Paul.
It wasn't until the cross that "God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them;" (2 Corinthians 5:19)
The reconciling the world by the finished cross-work was a truth hidden and kept secret before being revealed to Paul.
When we understand that God magnifies His word above His own NAME, (Psalm 138:2) we understand the importance of recognizing that "the gospel of the kingdom" and "the gospel of Christ" have different names for a reason. They're DIFFERENT.
I had a listener ask, "Since it is called the gospel of Christ, wouldn't that mean the gospel Christ preached?"
Good question, but the answer is no. Just because it is called “the gospel of Christ” does not mean it's the gospel Christ preached to Israel. Remember it's the gospel OF Christ, not BY Christ. Not preached BY Him, but OF Him. The "gospel of Christ" was OF Christ, but preached BY Paul.
And it only stands to reason that Paul's "gospel of Christ" was not preached BEFORE the cross, because Paul's gospel is centered AROUND the cross. It could only be preached in its fullness after it was accomplished.
Paul's gospel was not revealed until AFTER the cross when Christ revealed it to him. Paul understood that his gospel was different and is why he called it "my gospel." Not once, twiced, but THREE times. (Romans 2:16, 16:25, 2 Timothy 2:8) A term no other apostle ever used.
1 Corinthians 15:1-4 is where we who were "without Christ," and "strangers" with "no hope," (Ephesians 2:12) learn that Christ died for OUR sins, was buried and has risen. And when a person hears this good news and through faith alone BELIEVES it, they're "sealed with that holy Spirit of promise...until the redemption". (Ephesians 1:13-14, 4:30)
For years I have offered $1000,00 to anyone that can show me this good news to US anywhere outside of Paul's thirteen letters.
(Romans - Philemon)
It has yet to be done, but feel free to give it a try.
I submit that being "sealed with the holy Spirit" after belief is only found in the writings of Paul, and NOT in the books of Matthew, Mark, Luke, or John. In those books, Christ never told anyone that their faith in the finished cross work alone would save and seal them "unto the day of redemption."
How could he? His cross work wasn't FINISHED. -He was still alive.
Faith without works and being sealed with the Spirit was NOT "the gospel of the kingdom."
*The gospel of the kingdom was for Jews, not Gentiles.
*Preached in synagogues, not churches.
*And required faith plus works, not faith alone.
The "gospel of the kingdom" included:
"Healing every sickness and every disease among the people."
Speaking "with new tongues," and handling "serpents."
Cleansing "the lepers," raising "the dead," & "cast out devils:"
Selling "whatsoever thou hast, and give to the poor,"
Finding the one who "sit in Moses' seat:" and keep the LAW!
The list above is in no way exhaustive, but gives you cursory analogy of some of the many things you CAN'T do or DON'T do.
You see, everything changes when you come to that point of your life when you want Factual Truth VS. The Vain Traditions of Men you've been settling for.
It's your church-tradition that taught you to cherry-pick only PARTS of "the gospel of the kingdom." And it's things like this that make the world look at us as hypocrites.
The answer is to rightly-divide and not cherry-pick. Looking at the bible IN context instead of OUT of context. Paying attention to
BY whom, TO whom, and for what purpose.
By whom = The author
To whom = The audience
What purpose = According to Prophecy or Mystery. (Compare Peter, Acts 3:21 with Paul, Romans 16:25) Under Law or Grace. (Compare Matthew 7:12 with Romans 6:14)
The Gospel that Christ instructed Paul to preach cannot be found anywhere else in the word of God, and is why Paul called it "my gospel" in Romans 2:16, 16:25, & 2 Timothy 2:8, and said that what he preached was "unsearchable riches" in Ephesians 3:8.
It was the apostle Paul who the Godhead selected "to make all men see what is the fellowship of the mystery, which from the beginning of the world hath been hid in God,"..
"Truth persuades by teaching, but does not teach by persuading."