I remember when I first heard the 'dual gospel/kingdom/not rightly dividing the word' doctrine years ago. Since I was a voracious reader of the Bible I was able to instantly recognize the error of it. The whole counsel of the scriptures reveals the falseness of that misguided doctrine.
Did you ever notice that the disciples had no understanding of Paul’s gospel before the cross, but yet preached the gospel of the kingdom? Did they preach something they themselves did not understand?
Did you ever compare Matthew 24:14 with Colossians 1:5-6?