The Gospel is the distinguishing mark between all other false religions

When the apostle speaks "of the testimony of God, the object of saving faith, he tells us, 'it is Christ crucified,' 1 Cor. 1. 23-24. This was the great intent of the prophecies, which spake of the sufferings of Christ, and following glory: 'For to him gave all the prophets witness, that through his name whosoever believeth in him; shall receive remission of sin.' Acts 10. 43. This therefore is that truth of the gospel which is specially witnessed unto, and whereby Christ says, 'his kingdom is advanced in the world.' And this may be further clear unto us if we consider,

1. That this is the distinguishing truth of Christianity, whereby it us differenced from mere natural religion, and from all religions in the world that any way compete with it. This is the great thing, the first thing, that any religion can purpose to sinful men, How they should be pardoned, reconciled to God, and justified in his sight? If the Christian religion differ from others in any thing, it is in this: they purpose other ways of coming into favour with God, and false ways; but Christ has made peace by the blood of his cross, and thereby reconciled all his people, of all nations, unto God in one body; and has come and preacher this peace to them that were afar off, and to them that were nigh, and this is the truth wherein the true God is gloriously manifested and distinguished from every false god." - John Glas, The Works of Mr. John Glas: In Four Volumes, pg. 109

The Kingdom of God is maintained by the Truth

The "kingdom is not set up, maintained, or advanced in the world, by human policy, but by the truth. It might be expected, that when the power of the sword is laid aside, there would be more need of this kind of wisdom. But instead of this wisdom, we have here the truth. They that were employed at first in setting up and promoting this kingdom in the world, used great plainness of speech, and spake naked truth without disguise." - John Glas, The works of Mr. John Glas: In Four Volumes, pg. 100

No compromise between what the gospel is and isn't

The Bible makes no compromise on this point. There is no middle path between truth and falsehood. It is either truth or a lie! There is a great temptation to consider people who are pious, and faithfully attending services, as righteous. God has broken in me the deep rooted, but pernicious, inclination to self-redemption which runs so deep in man. We are born with it. I do not believe that there is a man who of himself wants to live by 'grace alone'. We secretly hope that there may still be something good in us. We are too proud to admit the contrary. The Bible breathes an atmosphere of sovereign grace. The sinner is justified by grace, through faith. Human cooperation in this work of justification is totally excluded. I am glad that God has revealed this truth to me. 'And ye shall know the truth, and the truth shall make you free' (John 8:32). - Toon Vanhuysse, The Truth Set Me Free, found in "Far From Rome Near to God, ed. Richard Bennett and Martin Buckingham, pg. 208 - 209