The gospel is a spiritual message conveyed by the Holy Spirt. It is an imbedded DNA that transcends time and space that is only effectual to the elect of God unto salvation. To the reprobate, it conveys condemnation and death. Consider the gospel message that was ‘preached to Abraham’. The gospel message that the Apostles preached while they walked w/ Christ. The message that we proclaim in this age. There is only one gospel and one truth; hence, it has to be appreciated spiritually and the message must be of Messiah.
The external call of God has changed over the centuries, given the circumstances. For example, what was proclaimed to Abraham, Isaac, Jacob and Noah (to name a few) was most likely different from the gospel proclamation of the Apostles while they walked w/ Christ and most definitely different in obvious fashions than the message we proclaim in this age. You might say, “Scott, all of these messages were in essence, ‘There is a Messiah and if you repent and believe, you can be saved!’ This is true, however, it has developed based on obvious unfolding of truth.
However, the internal call, that which the Holy Spirit brings to the elect, has been unchanged in all epochs. In that way, the gospel has not changed. That which men can mess up, the Holy Spirit perfects. Men can only be saved by the true gospel. This is exactly why many of us were saved under false teachers and aberrant presentations of the gospel message. This is exactly why, God continues to save people in these settings. When the actual word of God is read, no matter how many men mess up an orthodox theology, The Holy Spirit uses the real word of God to save.