The word gospel entered Old English as a translation of a Latin phrase meaning “good tidings” or “good news”: god (good) + spell (speech, message).
“The Gospel” (capitalized) is the “good news” that Jesus Christ redeemed mankind from sin and death.
“A gospel” is one of the four books in the New Testament (Matthew, Mark, Luke, John) that record the life and teachings of Jesus.
By extension, gospel may refer to the entire body of religious doctrine taught by Jesus Christ and the apostles. The word is also used to refer to all the books of the New Testament, or even to all 66 books in both testaments, from Genesis to Revelation.
In a broader sense, any heavenly revelation may be called a gospel:
God writes the Gospel not in the Bible alone, but also on trees, and in the flowers and clouds and stars. –Martin Luther.
A gospel may be any secular philosophy that serves as a guide to human conduct. A doctrine or opinion that is strongly held and preached with a religious fervor is also referred to as a gospel. For example:
I wouldn’t really advocate going to other sites and preaching the gospel of feminism to those that didn’t ask for it.
When they hear someone like me preaching the Gospel of firearms, they assume that I’m some kind of radical who wants everyone to be armed.
The phrase “The Gospel According to” is an extremely popular title opener:
The Gospel According to Coco Chanel
The Gospel According to Disney
The Gospel According to Thomas Jefferson, Charles Dickens and Count Leo Tolstoy
The Gospel According to Dr. Seuss
The Gospel According to Yoda
The psychologist William James wrote an essay called, “The Gospel of Relaxation,” and Atlantic journalist Ta-Nehisi Coates has written an article with the title “The Gospel of Rudy Giuliani.”
Finally, there are some idioms that employ the word gospel:
the social gospel
The application of Christian ethics to social problems like poverty and crime.
the gospel of wealth
The belief that the accumulation of vast wealth by a few is beneficial to all of society.
the gospel side
The north side of a Christian church, to the left of the main altar. The person reading the day’s gospel selection stands there.
A revivalist preacher who has a lively delivery.
A music genre that combines dominant vocals and strong harmony with Christian lyrics.
the gospel truth
Unimpeachable truth. For example, “I didn’t steal your lunch, and that’s the gospel truth!”
While not an idiom, the phrase “the synoptic gospels” is frequently seen or heard in discussions of the Christian New Testament. Of the four gospels that focus on the life and teachings of Jesus, three contain such similar content–in almost the same order– that they can be studied in parallel editions. The synoptic gospels are the books of Matthew, Mark, and Luke.