Most “evangelical” and “Baptist” churches today either preach no repentance or a false repentance, so a false gospel (2 Cor 11:4; Gal 1:6-9). Preaching that Jesus died for our sins, was buried and resurrected, is certainly the heart of the gospel (1 Cor 15:1-4), but that is not all there’s to it; there is more to the gospel than that. That passage does summarize it in a nutshell for a specific purpose as that entire chapter puts forth.
How preachers often use 1 Cor 15:3-4 today is not what Paul intended with this brief summary. It was given in the context of the resurrection of the body and warning of false teachers denying the resurrection. It wasn’t meant to be an all-inclusive account of the gospel. The Greeks rejected bodily resurrection and the false teachers at Corinth sought some means to consolidate rejection of bodily resurrection with the gospel. Paul is saying it can't be done. If you reject bodily resurrection, then you reject the resurrection of Jesus Christ, which then means you reject the true gospel, since this is part of the gospel.
What isn’t mentioned in 1 Cor 15:3-4 but yet taught in the Bible as part of the gospel includes the following:
sufferings of Christ (Is 53:5-7; Lk 24:46; 1 Pet 3:18);
the shedding of Christ’s blood (atonement) for our sins (1 Jn 2:1-2; Is 53:4-12; Heb 9:14-22);
the fear of God (Ac 13:6; Rev 14:7; Ps 66:16; Pr 1:7; 14:27);
the judgment of God (Ac 17:30-31; Rev 14:7);
repentance of sin and self and faith in the Lord Jesus Christ (Ps 2:12; Mk 1:1-4; Matt 4:17-23; Ac 20:21,24; Lk 24:44-48; 1 Cor 15:1-2);
worshipping God in spirit and in truth (Jn 4:23-24: Rev 14:7);
glorifying God (Rev 14:7; 16:9; Gal 6:14; Eph 1:6,12-14);
receiving an eternal inheritance (Heb 9:15) and eternal life (Jn 3:15-16,36; 5:24).
The gospel is read in many other passages such as Gen 15:1-6 (Abraham’s salvation, referenced in Rom 4:1-8, 18-25; Gal 3:6-9) and Ps 2:12 which reads, “Kiss the Son, lest he be angry, and ye perish from the way, when his wrath is kindled but a little. Blessed are all they that put their trust in him.” Repentance is seen in the first half here and faith in the last half. We see in the passage the truth of Ac 20:21, “Repentance towards God and faith toward our Lord Jesus Christ.” The foundation of gospel preaching is repentance as seen by God the Son (Matt 4:17; Lk 24:44-48), which is granted by God to sinners (Rom 2:1-4; Ac 5:31; 11:18) which without no man can be saved (Lk 5:31-32; Matt 21:28-32).
Gospel/salvation (same thing) preaching without repentance is “another gospel” (2 Cor. 11:4), “Which is not another; but” a “pervert[ed] . . . gospel of Christ.” (Gal 1:6-7). A person can’t and won’t be saved when they embrace a false gospel, and true saving repentance is most definitely part of the gospel, as made very clear by Jesus (see Matt 4:17; 21:28-32; Ac 26:18-20; Lk 5:31-32; 24:44-48), by Paul (see Ac 20:21,24; 26:18-29), by Christs commission of the apostles (compare Matt 10:6-7 and Mk 6:12), and by John the Baptist (see Mk 1:1-4; Lk 3:1-16; Matt 3:1-10). We also see this truth throughout the gospels in Jesus’s preaching to His mostly lost audience (e.g. Matt 11:20-30; 12:38-42; Mk 8:34-38; 10:17-21; Lk 14:25-32).
Today many gospel minimalists preach that repentance is not part of the gospel to avoid being condemned a false teacher preaching a false gospel since their preaching is largely absent of repentance. And not to mention their perversion of this doctrine when they do rarely allude to it and their denial of its absolute necessity for salvation and immediate and ongoing fruit from true conversion. But in their craftiness, they still expose a false gospel (Gal 1:6-9) which means they are false teachers (2 Pet 2:1). The true minister of God however has,
"renounced the hidden things of dishonesty, not walking in craftiness, nor handling the word of God deceitfully; but by manifestation of the truth commending ourselves to every man's conscience in the sight of God." (2 Cor 4:2).
In a nutshell, saving repentance is a change of the mind and the will concerning ones ungodly and wicked state, wrought in the poor, broken and contrite heart of a sinner, so as with godly sorrow and self denial (dying to self, losing your life, taking up the cross) completely turning from their sin, their selves, their stuff and their people, i.e., forsaking all, and turning to God by faith in the Lord Jesus Christ and His Word, in surrendering to Him as Lord and King. This truth is seen throughout Scripture, in doctrine, the commands, and preaching. For instance: Is 55:1-7; 66:2; Ezk 14:6; 18:27-28,30-32; 33:9-11; Ps 2:12; 34:18; 51:17; Matt 6:24; 10:32-39; 21:28-32; Mk 1:1-4; 8:34-38; 10:21; Lk 3:3-16; 5:1-11, 27-32; 57-62; 12:8-9; 13:1-5, 23-30; 14:16–15:32; 17:26-33; 18:9-17, 18-30; 19:1-10; 20:17-18; Jn 12:24-26; Ac 3:19; 17:30-31; 19:17-20; 26:20; 1 Th 1:9-10; Rev 9:20-21; 16:9-11.
“Seek ye the Lord while he may be found, call ye upon him while he is near: Let the wicked forsake his way, and the unrighteous man his thoughts: and let him return unto the Lord, and he will have mercy upon him; and to our God, for he will abundantly pardon.” (Is 55:6-7).