Adam Clarke on Psalms 31:23

O love the Lord, all ye his saints - It is only the saints that can love God, as they only are made partakers of the Divine nature. Holy spirits can love God, who is the fountain of their holiness; and the saints should love him.

Preserveth the faithful - Those who, being filled with the love of God, bring forth the fruits of that love - universal obedience to the will of God; for to such persons his commands are not grievous, their duty is their delight; while a man is faithful to the grace he has received, that is, uses and improves the talents with which God has intrusted him, God's service is perfect freedom.

The proud doer - The man of the proud heart, haughty and supercilious carriage, and insulting and outrageous conduct. A proud man is peculiarly odious in the sight of God; and in the sight of reason how absurd! A sinner, a fallen spirit, an heir of wretchedness and corruption-proud!

Proud of what? Of an indwelling devil! Well; such persons shall be plentifully rewarded. They shall get their due, their whole due, and nothing but their due.