According to the NT (esp. in Paul) and OT, the gospel is simply the incarnational and atoning work of the Son. The work of man in his faith-act, repentance, obedience, etc. is the “result” and not the substance of the gospel. In other words, the gospel has nothing to do with man, rather, all to do with the Son.
The gospel is not limited to one doctrine, such as election (as many overly zealous, yet unripe, Christians assume), rather, the gospel is the work of the Son consisting of both His Humiliation (incarnational work, life, suffering, death, being buried) and His Exaltation (resurrection, ascension, seated at the right hand of God, second coming).
Paul clearly summarizes his gospel of the Son definition in esp. in such places as Rom. 1:1, 3; 1 Cor. 15:1-4; and 2 Tim. 2:8 (see below). However, in many other places, the apostle provides a positive detailed delineation of the gospel—namely, the Son’s incarnational and cross work, even without using the term “gospel” (cf., Rom. 5:1, 10; 8:32; 9:5; 1 Cor. 2:8; 2 Cor. 8:9; Gal. 3:13; Eph. 5:25; Phil. 2:6-11; Titus 3:5-7 et al.).
The Gospel is the work of God the Son
Rom. 1:1, 3: “the gospel of God. . . . regarding His Son”
1 Cor. 15:1-4 (A.D. 54):
“Now I make known to you, brethren, the gospel [euaggelion] which I preached [euēggelisamēn- aorist ind. of euaggelizō], which also you received in which also you stand, 2 By which also you are saved, if you hold fast the word which I preached [euēggelisamēn– aorist ind.] to you, unless you believed in vain. 3 For I delivered to you as first importance [prōtos] what I also received that Christ died for our sins according to the Scriptures, 4 and that He was buried, and that He was raised on the third day according to the Scriptures.”
2 Tim. 2:8: “Remember Jesus Christ, risen from the dead, descendant of David, according to my gospel.”
So Rom. 10:15: “How will they preach unless they are sent? Just as it is written [Isa. 52:7], ‘How beautiful [hwraios, ‘timely’] [are] the feet of those gospelizing [euaggelizomenwn] good things.”