For the sake of brevity one thing was not expounded on in "What is the Gospel?". There is a key component upon which a correct understanding of the gospel stands or falls: sin. Without a correct view of sin the gospel looses its power.
Sin is best described, according to the Westminster Shorter Catechism, is want of conformity unto or transgression of the Law of God. This definition is as condensed a view of sin as possible, but there is more.
In the pages of Scripture sin is described as: whoredom (i.e. adultery; Hosea 1:2), an egregious affront to God's eyes (Habakkuk 1:13), treason (Ibid.), et al. Furthermore, it is said of those who have sinned that they are: of a debased mind (Romans 1:28), unrighteous, evil, covetous, malicious, slanderers, haters of God, inventors of evil, foolish, faithless, heartless, ruthless (Romans 1:29ff).
How then can anyone have a low view of something the Bible takes so seriously? The answer to that question lies in their own sinfulness; for all have sinned (Romans 3:10ff) and no one is good and our minds are twisted and enslaved (Romans 7:19). Therefore, for the gospel of Jesus Christ to show the gravity of what occurred on Golgotha two thousand years ago, one must understand the situation that all men are in.
In the Garden of Eden it was promised Adam that on the day he ate of the Tree of the Knowledge of Good and Evil he would surely die (Genesis 2:17), but Adam and Eve did not die that day, at least not physically but spiritually. The Apostle Paul words the human condition thus:
"And you were dead in transgressions and sins 2 in which you once walked following the course of this world, following the prince of the power of the air (i.e. the devil), the spirit that is now at work in the sons of disobedience (i.e. mankind)-..." (Ephesians 2:1-2, emphasis and parentheses added)
Sin is the reason that Jesus Christ died on the cross. Sin is the reason that a holy man, the only Son of God (Philippians 2:6), was falsely tried and convicted and murdered at the hands of those he had come to save (Luke 23ff). Sin is the reason that a Holy God poured out the full measure of his wrath upon His only Son to save sinners (2 Corinthians 2:16ff; Colossians 2:13). Therefore, any view that is less than that of the Bible and therefore God, robs the gospel of its power.