Faith, Holiness, Assurance, and the Covenant of Grace

"This covenant [of grace] does not only give life upon terms of believing, but faith also and holiness the necessary means of attaining that [eternal] life. And this not upon your ingenious [sic, for "ingenuous"] compliance, as some term it, or better improvement of what you have in common with other men, (such allegations the Lord disallows, and often cautions against), but of grace. It is a covenant made up of promises, and promise, by scripture intention, is always free, both freely made and freely performed, without the desert or procurement of men." --Elisha Coles, "A Practical Discourse of God's Sovereignty" 

The difference between biblical Christianity and any of its competitors is that it stands on that great principle: Justification is by grace alone through faith alone in Christ alone for the glory of God alone. Every other religion, whether it falsely claims to be Christian or it is a distinctly-named religion, makes salvation based on the good works of the supplicant, whether in part or in whole. 

That is because the true Christian is aware that he has nothing in himself by which he can appeal to God as worthy of salvation. In answer, as the Puritan Coles says above, Jesus purchased everything, not just justification itself, but also the faith with which we respond and the holiness which it produces. By grace, He purchased them on the cross for everyone that the Father had given Him to be redeemed, and His blood cannot fail to achieve its purpose (John 6:37-39).

In contrast, the unbeliever is dead in trespasses and sins (Ephesians 2:1, Romans 3:23), and thereby under the just wrath of God (Ephesians 2:3, Romans 6:23). It is impossible for such a wretch to offer anything of himself to the offended God, because only a believer can do anything of spiritual good (Romans 14:23, Hebrews 12:14). The Father, apart from the intercession of the blood of Jesus, refuses even to look in love on such a one: "[God's] eyes are too pure to look on evil, and You cannot look on wickedness" (Habakkuk 1:13). Alas, even the prayers of the unbeliever are repugnant to Him (Proverbs 28:9). 

For those for whom Jesus died, that holy flood washed away our sins, the judgment for our sins, and purchased for us faith and holiness. It is only in Him that we can have hope, and have assurance that our hope is on a sound foundation.