To "grudge not" is to "complain not." We are not to groan or grumble against each other. In this life, where there is the tendency to be partial to others who have more than we do, James reminded us not to be grumbling and complaining about others.
We are not to develop an attitude of thinking the other person always has it better than we do. We must remember that the Lord can come back at any time to judge us for such attitudes.
James said, "The judge standeth before the door" (James 5:9); in other words, He is ready to enter the door. Having this concept of the soon return of Christ keeps us from being so critical of each other.
Always remember, Christ is about to enter the door, so the words you speak should be wholesome rather than negative or hurtful. So realizing that the Lord may soon return will have a significant effect on our attitudes toward others.
We will not be so quick to criticize others when we realize the Lord is about to return to judge us for what we have said. The Lord is fully able to judge the motives of our hearts (see 1 Cor. 4:5), but we must never take that prerogative upon ourselves.
The Lord will judge not only the good and bad things we have done but also our good and bad attitudes. If you have been mistreated and have borne up under it in a way that glorifies the Lord, you may be assured of a reward.
If you have been neglected by friends and by the world, the Lord will take care of that also. Let us trust Him completely to reward as He sees best because He is absolutely just and loving.
"And every man shall receive his own reward according to his own labour" (1 Cor. 3:8).