“Wait for the Lord;
be strong, and let your heart take courage;
wait for the Lord!”
Psalm 27:14

Conscious of his own weakness, David addresses himself (the verb is singular), and seeks to encourage his confidence in God. The word wait suggests that there are further trials still to come, therefore he sets before himself the cross he must bear. We wait upon God, therefore, when he permits us to languish under afflictions. It is as if David is saying: “When I am afraid and temptation stalks me, I must not faint, but must determine to remain resolute in my mind.” Yet in his weakness, through prayer he borrows strength from God, the Holy Spirit. Because the trials of the saints are often protracted for a long time, he again exhorts himself, wait for the Lord. 

—John Calvin, Commentary on Psalms