The gospel preached by the Apostles HAD REFERENCE TO THE SAME THING - that is The Kingdom of God

But when they believed Philip preaching the things concerning the kingdom of God, and the name of Jesus Christ, they were baptized, both men and women. (Acts 8:12,25).

And 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 19:8).

And now, behold, I know that ye all, among whom I (Paul) have gone preaching the kingdom of God, shall see my face no more. (Acts 20:25).

And Paul dwelt two whole years in his own hired house... preaching the kingdom of God, and teaching those things which concern the Lord Jesus Christ, with all confidence, no man forbidding him. (Acts 28:30,31).