31 • "When the Son of Man comes in his glory,
• and all the angels with him,
• then he will sit on his glorious throne."
The gloriously eternal God created both humans and angels for his own glory. Some of the angels and all of mankind fell short of God's glory because of the entrance of sin. In accordance with God's counsel, the Son of God, also eternally glorious (… not count[ing] equality with God a thing to be grasped,[Philippians 2:6]) left heaven's glory in order to restore fallen humanity through his death on the cross. On his being raised from the dead he returned to heaven with the promise that he would return in glory.
Here we have the tripartite glory of the returning Christ displayed. Two parts of that glory will be complete on his return (his own glory as the Son of Man, the judge and the glory of his glorious throne, the seat of judgement.) The third part of his glory will be to be completed, not that the angels are not glorious in themselves, but that this glory of his will be made complete when to the number of the angels will be added those (both male and female) adopted from among mankind to be God's heirs who, with Christ and the angels will make up the number of the sons of God at last.
The company of the holy, elect angels, saved from ever falling into sin will be joined by the complete company of holy, elect people redeemed from sin by the glorious death on the cross of him, who knowing no sin was made sin for us.
For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God. [2 Corinthians 5:21]