That they did is a central tenet of a certain form of dispensationalism sometimes called “ultra”-dispensationalism. Here is an excerpt from a blog post at: Did Paul Preach a Different Gospel? – Escape to Reality

Read the New Testament and you might come away with the idea that there is more than one gospel.

The very first words of the New Testament in the King James Bible are, “The Gospel According to Matthew.” Read on and you will also find the gospels according to Mark, Luke, and John.

In the Gospels of Matthew and Luke, we find Jesus preaching the gospel of the kingdom, while Mark refers to the gospel of Jesus Christ and the gospel of God.

The word gospel does not appear in John’s Gospel, but in Acts and all the letters that follow, the gospel is mentioned plenty of times with different labels:

• the gospel of Jesus Christ (Mark 1:1)
• the gospel of Christ (Rom. 15:19, 1 Cor. 9:12, 2 Cor. 2:12, 9:13, 10:14, Gal. 1:7, Php. 1:27, 1 Th. 3:2)
• the gospel of God (Mark 1:14, Rom 1:1, 15:16, 2 Cor. 11:7, 1 Th. 2:2, 8, 9, 1 Pet. 4:17)
• the gospel of the blessed God (1 Tim. 1:11)
• the gospel of his Son (Rom 1:9)
• the gospel of the kingdom (Matt. 4:23, 9:35, 24:14, Luke 16:16)
• the gospel of the glory of Christ (2 Cor. 4:4)
• the gospel of your salvation (Eph. 1:13)
• the gospel of peace (Eph. 6:15)
• the glorious gospel of the blessed God (1 Tim 1:11)
• the eternal gospel (Rev. 14:6)

These are not different gospels but different labels for the one and only gospel, namely the gospel of grace.

You are invited to read the entire article.

There is a detailed response to the topic of ultra-dispensationalism that can be read at: Wrongly Dividing the Word of Truth – Table of Contents – Bibleline Ministries

Be Blessed!