The greatest sin is unbelief. It is unbelief that Jesus Christ actually accomplished everything for us on the cross, and that we can only have hope and rest in that alone.
Charles Spurgeon writes in one of my favorite chapters of any book ever called 'Salvation Altogether by Grace' (Grace: God's Unmerited Favor)...
"The purpose of God is not founded on any foreseen merit of ours, but upon His grace alone. It is grace, all grace and nothing but grace from first to last. Man stands shivering outside, a condemned criminal, and God, sitting upon the throne, sends the herald to tell him that He is willing to receive sinners and to pardon them. The sinner replies, 'Well, I am willing to be pardoned if I am permitted to do something in order to earn pardon. If I can stand before the King and claim that I have done something to win His favor, I am quite willing to come.'
However, the herald replies, 'No, if you are pardoned, you must understand that it is entirely and wholly as an act of grace on God's part. He sees absolutely nothing good in you. He knows that there is nothing good in you. He is willing to take you as you are - black, bad, wicked and undeserving. He is willing to give you graciously what He would not sell you for any price (Isaiah 55:1) and what He knows you cannot earn from Him. Will you receive it?'
In the natural state, every man says, 'No, the very idea is abhorrent to me. I will not be saved in that style.' Well then, misguided soul, remember that you will never be saved at all, for God's way is salvation by grace. If you ever are saved, my dear one, you will have to confess that you never deserved or merited one single blessing from the God of grace."
The temptation to want to earn our salvation is the cancer within every one of us. It points to self, and completely goes against the purpose of salvation in the first place: the Glory of God. My tendency to neglect God's gift, more bluntly to not accept it for what it is, eats away at me. It tells me I haven't done enough today to be favorable for the King. It demands me to try harder, when really I have nothing to offer. However, Jesus demands nothing of me, but to receive Himself. And it doesn't even depend on the 'excellency in my reception' or how well I trust, but rather in who I am simply trusting in. Jesus. Period. May we all further know the fullness of His grace.