Gospel,means Good Tidings, especially the good news concerning Christ, the Kingdom of God, and Salvation; hence the teachings of Christ.
For our purpose here, this definition, though a brief one, is ample. Christ says that if the Church is built upon My Gospel, that is, if it is built upon My teachings, I will recognize it as My Church. And further He says, "If ye call upon the Father, for the Church, if it be in My Name the Father will hear you; And if it so be that the Church is built upon My Gospel then will the Father show forth His own works in it." (vv. 9-10)
But on the other hand, if a church is built upon the works of men, or upon the works of the devil, that church may bring its workmen joy for a short season, a sort of personal gratification that feeds the hunger of greed and infatuation, but however, as unchangeable as is the rising Sun, the events of time await only the grace of God. Therefore, all their works "shall be as the morning cloud, and as the early dew that passeth away...and as the smoke out of the chimney." (Isaiah 5:24)
Man and his works, and the works of Satan, go hand in hand, and are fit only "to be hewn down and cast into the fire." (v. 11) It is, moreover, that the works of men and the works of devils condemn the doers thereof, and because their works are evil, their works and they themselves will be hewn down and be consumed by fire!