The author of Hebrews in 12:15-17 warns:
Looking diligently lest any man fail of the grace of God; lest any root of bitterness springing up trouble you, and thereby many be defiled; Lest there be any fornicator, or profane person, as Esau, who for one morsel of meat sold his birthright. For ye know how that afterward, when he would have inherited the blessing, he was rejected: for he found no place of repentance, though he sought it carefully with tears.
C. H. Spurgeon wrote concerning the failing of the grace of God:
Under the means of grace, there are many who do “fall short of the grace of God.” They get something that they think is like grace, but it is not the true grace of God, and they ultimately fall from it, and perish. . . . [I]n church fellowship we ought to be very watchful lest the church as a whole should fail through lack of the true grace of God, and especially lest any root of bitterness springing up among us should trouble us, and thereby many be defiled. We must remember that though we are saved by grace, yet grace does not stupefy us, but rather quickens us into action. Though salvation depends upon the merits of Christ, yet those who receive those merits receive with them a faith that produces holiness.
Spurgeon explains that this "failing" is "falling short," and then "falling short" is not getting "the true grace of God" but "something that they think is like grace." He says the true grace of God "does not stupefy us, but rather quickens us into action." How you know you didn't get the true grace of God is that the placebo does not produce holiness.
The grace of God is what saves us. Very often people want that base covered, but at the same time they don't want the holiness true grace produces. Hebrews uses Esau as an example. He allowed his fleshly desire to keep him from true grace, replacing it with something short of God's grace. God's grace produces holiness.
Through the years, I've read many different opinions about what the "root of bitterness" is. In the context, it's a cause for failing of the grace of God. Some say that the root of bitterness is an apostate in the church, like Esau, who brings about further apostasy from others. Others say that it is sin, which is bitter and defiling. Rick Renner writes:
"It" pictures a person who is continually troubled, harassed, and annoyed by thoughts of how someone else wronged him. The offended person is now so troubled that he is almost emotionally immobilized. Instead of moving on in life, he gets stuck in the muck of that experience, where he wallows day after day in the memories of what happened to him. If that person doesn't quickly get a grip on himself, he will eventually fulfill the next part of the verse.
Tozer explained it the same way:
The sad and depressing bitter soul will compile a list of slights at which it takes offense and will watch over itself like a mother bear over her cubs. And the figure is apt, for the resentful heart is always surly and suspicious like a she-bear!
Perhaps the preceding verse, verse 14, gives a clue:
Follow peace with all men, and holiness, without which no man shall see the Lord.
Esau lacked peace between he and his father, Isaac, and his brother, Jacob. So many especially today allow the slights, real and otherwise, and even actual sins against them to keep them from the grace of God. They also often use these temporal affronts to justify their lusts, incongruous with the true grace of God. It ultimately reflects on their view of God and His goodness to them.
Spurgeon assessed failing of true grace comes by replacing it with something short of the grace of God. I'm titling what I believe is the most common contemporary replacement for true grace, "postmodern grace" (Jesus Loves Me with postmodern lyrics). It isn't the grace of God, because it is short of the grace of God.
Postmodern truth is your truth. Postmodern grace is your grace. It doesn't follow peace, because it allows a grudge and resentment to keep it from that. It doesn't follow holiness, because it sells holiness for temporal, carnal appetites, like the morsel of Esau. It counts this though as the grace of God. Postmodern grace isn't about pleasing God, but about pleasing self. Postmodern grace self-identifies as grace, which is in fact moral relativism. It doesn't follow after holiness, but after its own lust.