Biblically speaking, the gospel (good news) is the substitutionary and sacrificial work of Christ—not the work of man in his response, faith, repentance, good behavior, etc. Besides passages such as 1 Corinthians 15:3-4, which we will deal with shortly, Paul makes this point clear in Romans 1:1, 3, “The gospel of God . . . concerning His Son.” So, the gospel in and of itself has nothing to do with man, but everything to do with the atoning work of Jesus Christ, God the Son. We must not confuse the work of Christ, which is the gospel—the good news of Jesus’ cross work—with the response of faith in Christ, repentance, obedience, etc. Salvation is solus Christus (through Christ alone), thus, Hs work being the very ground or cause of justification, and faith being the very alone instrument.
The gospel then is comprised of all essential theology of the Christian faith since it involves the person, nature, and finish work of Christ. Simply, the gospel is the atoning work of God the Son, in incarnation, death, and resurrection. And trusting Him alone for salvation (Rom. 10:9, 13; 1 Cor. 15:3-4 [see discussion below on this passage]; 2 Tim. 2:8).
In expanded detail, the essential doctrines of the Christian faith include:
- The person of the Son is truly God and truly man, the two natured person—being distinct from the Father who sent Him (John 1:1, 14, 18; 5:17-18; 20:28; 1 Cor. 2:8; Rom. 9:5; Phil. 2:6-8; Col. 2:9; Heb. 1:3; 1 John 4:2-3; 5:20; Rev. 1:7-8).
- The sending of the Son to earth from the Father out of heaven (John 3:13, 16-18; 6:38; 16:28).
- A literal descendant of David, born of a virgin (2 Tim. 2:8; Matt. 1:18; Rom. 9:5; Gal. 4:4).
- The perpetual (ongoing, permanent) incarnation of the Son—the Word became flesh (John 1:1, 14; 2 Tim. 2:8; 1 John 4:2-3; 2 John 1:7).
- The Son’s substitutionary (vicarious) atoning sinless life (preceptive obedience) and cross work (penal obedience) as the very ground of justification, which removed the sin-guilt and God’s wrath due to us for our sins (Gen. 15:6; Isa. 53:11; Mark 10:45; John 6:37-39; Rom. 5:6, 8, esp. v. 10; 8:32; 1 John 2:2, 4:10).
- Salvation (justification), then, is through faith alone “apart from works” (Acts 10:36, 43; Rom. 4:4:4-8; 5:1; Eph. 2:8-9; 2 Thess. 2:13; 2 Tim. 1:9).
- Jesus’ real death and physical resurrection (John 2:19-21; 19:30; Acts 1:11; 17:31; Rom. 10:9; 1 Cor. 15:3-4; Titus 2:13).
- His accession to the Father (John 6:62; 16:10, 28; 20:17; Acts 1:10-11; Heb. 10:12-13).
- His (physical) second coming (Acts 1:10-11; Titus 2:13-14; 1 John 2:28).
- The concept of the Trinity—namely, one true eternal God revealed in three distinct persons (see chap. 3 above).
The person (unipersonal, i.e., distinct from the Father, and Holy Spirit), nature (truly God truly man) and finished completed work (justification through faith alone) are necessary and indispensable to the Christian faith. They also imply other important doctrines such “total inability,” that is, in man’s unconverted spiritual state he cannot (no ability) please or come to Christ (John 6:44; 8:43-44, 47; Rom. 3:10-18) due to the inherent sin-guilt (imputed sin) of all men resulting from the first sin in the Garden. These doctrines constitute the key ultimate test in which distinguishes genuine Christianity from false non-Christian (atheistic) religious cults and world religions.
All must be affirmed in a basic sense, and none can be denied. Further, one cannot affirm some of these, but not the others. For example, Roman Catholicism (as discussed below) officially embraces the Trinity, deity of Christ, the incarnation, virgin birth, and Jesus’ resurrection. However, because Roman Catholic doctrine rejects that the alone work of Christ is the absolute and sufficient means and ground of justification, Rome falls outside of Christian orthodoxy (cf. Gal. 1:6, 8)—hence, non-Christian.
Thus, it is not the Jesus of biblical revelation that Rome embraces, rather a different Jesus and a “different gospel.” Therefore, all things pertaining to the gospel are “essential” theology. Whereas secondary theology is any doctrine that is not essential to one’s salvation—namely, any doctrine that does not fundamentally deny or distort the nature and/or finished work of Christ (e.g., the OT Law, spiritual, gifts, method of water baptism, eschatology [i.e., end-time teachings], etc.). Again, the sufficiency of the gospel is the work of the Christ. and justification through faith alone is the only recognized gospel.
 “Remember Jesus Christ, risen from the dead, descendant [spermatos] of David, according to my gospel.”