O love the Lord, all ye his saints - This is the “application” of all the truths suggested in the psalm. The experience of the psalmist had shown the wisdom of trusting in God in times of danger and trouble, and had laid the foundation for a proper exhortation to others to imitate his example; an argument why all the people of God should love him, and should be of good courage. The reason here assigned for their loving the Lord is, that he preserves those who are faithful to him, and “rewards the proud doer.” This is a reason for loving God, or for putting our trust in him, though the psalmist does not say that this is the only reason for doing it. The meaning here is, that the dealings of God toward the psalmist had established this truth in regard to the character of God, that he does preserve the faithful, and does punish the proud, and that this fact constitutes a reason why all his people should confide in him.
For the Lord preserveth the faithful - The faithful; those who put their trust in him; those who do not give up in despondency and despair in time of danger and trouble; those who do not forsake him even though for a time he seems to forsake them. What God looks for mainly in his people is confidence; faithfulness; trust; fidelity.
And plentifully rewardeth - “Abundantly” rewards. Literally, “in plenty.” That is, his punishment does not fall short of the desert of the wicked man. It is ample or full. He does full justice.
The proud doer - “The man working pride.” The reference is to the man who is confident in himself; who seeks to aggrandize himself, and who in doing this is regardless of the rights of others.