Isaiah 6:1-4
And one cried unto another, and said, Holy, holy, holy, is the Lord of hosts: the whole earth is full of his glory. Isaiah 6:3. CC 233.1
These holy beings sang forth the praise and glory of God with lips unpolluted with sin. The contrast between the feeble praise which he [Isaiah] had been accustomed to bestow upon the Creator and the fervid praises of the seraphim, astonished and humiliated the prophet.... CC 233.2
While he listened to the song of the angels, ... the glory, the infinite power, and the unsurpassed majesty of the Lord passed before his vision, and was impressed upon his soul. In the light of this matchless radiance that made manifest all he could bear in the revelation of the divine character, his own inward defilement stood out before him with startling clearness. His very words seemed vile to him. CC 233.3
Thus when the servant of God is permitted to behold the glory of the God of heaven, as He is unveiled to humanity, and realizes to a slight degree the purity of the Holy One of Israel, he will make startling confessions of the pollution of his soul, rather than proud boasts of his holiness. In deep humiliation Isaiah exclaimed, “Woe is me! for I am undone; because I am a man of unclean lips....” This is not that voluntary humility and servile self-reproach that so many seem to consider it a virtue to display. This vague mockery of humility is prompted by hearts full of pride and self-esteem. There are many who demerit themselves in words, who would be disappointed if this course did not call forth expressions of praise and appreciation from others. But the conviction of the prophet was genuine.20The S.D.A. Bible Commentary 4:1140. CC 233.4
The seraphim before the throne are so filled with reverential awe in beholding the glory of God that they do not for an instant look upon themselves with self-complacency, or in admiration of themselves or one another. Their praise and glory are for the Lord of Hosts.... They are fully satisfied to glorify God; and in His presence, beneath His smile of approbation, they wish for nothing more. In bearing His image, in doing His service and worshiping Him, their highest ambition is fully reached.21Ibid. CC 233.5