We've all heard the homespun wisdom "Patience is a virtue," which sounds almost mystical but is really like saying, "Blue is a color." The more interesting question is, what's a virtue? A virtue is a character trait that helps you achieve some desired good or outcome. Virtues are central to Christian theology. The speaker desires to serve God, but his impatience and sense of wounded pride threaten to get in his way by leading him to rashly criticize his "Maker." The virtue of patience helps him to remember that it's not all about him. Just because he thinks he has something to offer doesn't mean that God needs him to act right away.
The speaker's fault in the first section of the poem is not merely impatience; it is that he misunderstands the nature of God's justice.
The fundamental principle upheld by the poem is endurance of misfortune, of which patience is only a part.