Moreover, brethren, I declare unto you the gospel...by which also ye are saved...(unless ye have believed in vain)...how that Christ died for our sins according to the scriptures, and that He was buried, and that He rose again the third day according to the Scriptures; and that He was seen..." (1 Corinthians 15:1-8)
Today, we hear many say 'doctrine divides', so let's not emphasize 'doctrine'. "God is love, not doctrine". Let's just preach "the gospel", so that people get saved, and not over-emphasize doctrine. But this is a false dichotomy. It's not the gospel vs. doctrine. In point of fact, an immense amount of doctrine is included in "the gospel". Let's unpack the text of 1 Corinthians 15:1-8 and get a glimpse of all that is involved in this 'simple' Bible passage that defines "the gospel".
To begin with, the gospel is not a fable (cf. 2 Peter 1:16; a "fable" is a fictitious story invented to teach a moral lesson). The gospel involves historical events. Admittedly, it is the correct understanding/interpretation of those events that is critical, but that does not diminish the fact that the historicity of these events is necessary and indispensable. They must be real events that really happened: Christ died, He was buried, He rose again, He was seen in His resurrection body by many people. That Christ was buried is evidence to substantiate the fact that He really died; that Christ was seen is evidence to substantiate the fact that He really rose again! You do not include 'evidences' for a fable, only a real, historical event. In fact, Paul says that if the resurrection of Christ did not really and literally happen, our faith is "vain" (i.e., it accomplishes nothing!; 1 Corinthians 15:14). Since "the gospel" is based on an historical event, the Bible must be an infallible record of historical events, which demands that it be the inspired Word of God. Thus, the doctrine of the inspiration of Scripture is involved in "the gospel".
All have sinned, and there is a penalty for our sin. Furthermore, there is nothing we ourselves can do about it, which is why "Christ died for our sins". The penalty required for our sin is an infinite penalty, because we have sinned against God, Who is an infinite Being . The only way a finite being (i.e., a creature) can pay an infinite penalty is for that penalty to be enforced over an infinite period of time (i.e., eternal judgment). Thus, the doctrines of the nature of man and the nature of God is involved in "the gospel".
The historical fact that "Christ died for our sins" has to actually accomplish something. This is where the various theories of the atonement come into play. Christ's death on the cross was not just an expression of love, or an example for us to emulate. 1 John 2:2 makes clear that the death of Christ was a propitiation (i.e., substitutionary atonement) for the sins of the world. Propitiation means a payment that satisfies the debt. Thus, the doctrine of the substitutionary atonement is involved in "the gospel".
Continuing on, the hypostatic union is necessary. Christ must be a Man in order to die (God cannot die!). Christ must also be a Man to redeem mankind (i.e., qualification of a Kinsman Redeemer; cf. Hebrews 2:14-16). He must be a perfect, sinless Man to "die for our sins", else He would have to die for His own sins. However, if Christ is merely a Man, even a Perfect Man, for Him to pay the penalty for my sins, which involves an infinite penalty, He would have to suffer for an infinite period of time (forever); but in such a case, the penalty for my sins would never be paid (i.e., only in eternity), so it is no help. For me to be saved, I need Christ to be able to pay for my sins in a finite period of time. The only way for an infinite penalty to be paid in a finite period of time is for the One paying it to be infinite; an infinite Being can pay an infinite penalty in a finite period of time. For Christ to be an infinite Being, He must be God. Christ must be both a Perfect, sinless Man and God, at the same time. A corollary to this is that the only way to produce a God-Man is by a supernatural, virgin-birth. Thus, the doctrine of the nature of Christ (including the virgin-birth and the hypostatic union) is involved in "the gospel" (cf. 2 John 1:9).
Many professing Christians think the doctrine of the Trinity is irrelevant or unimportant. Many are even embarrassed by it. Admittedly, it is hard to understand, and it is even harder to explain to an unbeliever. Without the Trinity, however, "the gospel" unravels. On the cross, Christ (who is God) was paying the penalty for our sins. To whom was He making this payment? God. So, God was suffering for human sin, and at the same time God was judging human sin. Jesus cried, "My God, My God, why hast Thou forsaken Me?" If God is one Person, then He cannot be suffering for sin and judging sin at the same time. For this reason (among others), there must be a plurality of Persons within the Godhead. God the Son was suffering for human sin, and God the Father was judging human sin. Thus, the doctrine of the Trinity is involved in "the gospel".
Finally, even some eschatological doctrine is relevant to "the gospel". The Bible teaches clearly that the resurrected Jesus Christ will come again to judge the world (2 Timothy 4:1). Thus, a denial of the bodily return of Christ is a denial of coming judgment. It is this coming judgment from which "the gospel" saves us. Thus, even the doctrine of the return of Christ is involved in "the gospel".
Doctrine Is Important
The gospel cannot be separated from sound, Biblical doctrine. Implicitly loaded into "the gospel" is an immense amount doctrine which, if denied, renders one's faith vain.
Doctrines that are essential to "the gospel" have come to be called "the fundamentals". Historically, those who have sought to define and defend these fundamental doctrines have been known as 'Fundamentalists'. To compromise on a fundamental doctrine is to jeopardize "the gospel", and to deny a fundamental doctrine is to deny "the gospel". Fundamentalists refuse to extend Christian fellowship with those who deny a fundamental doctrine on the basis that they do not share the same understanding of "the gospel" (i.e., they are not Christians).
As this article has demonstrated, fundamental doctrines include the inspiration of Scripture; the virgin-birth, true humanity and undiminished deity of the Lord Jesus Christ; the substitutionary atonement of Christ; the bodily resurrection of Christ; and the bodily return of Christ to judge the world. These are not the only fundamental doctrines. However, every attempt to exhaustively enumerate the fundamental doctrines has failed, and today no one believes such an exhaustive list can be compiled. The reason for this is that our doctrinal understanding (i.e., clarification and articulation of doctrine) is constantly progressing, and it is doing so generally in response to new heresies that arise . As new heresies continue to arise today, our doctrinal understanding/clarification continues to be refined...
 As an illustration, consider the consequences if someone kills a mosquito, a dog, a baby—even we recognize there ought to be different penalties for these crimes. What makes the difference? The value of the person against which one commits the crime determines the just penalty.
 If you think of the historic creeds (e.g., Apostles, Nicean, Chalcedonian), they were continually revised and expanded with time in order to clarify points that were being challenged by heretics.