From The Gospel of Matthew, by Curtis Mitch and Edward Sri, commenting on Matthew 25:31-46
The opening verse sets the scene. It is the return of the Son of Man surrounded by heavenly angels and seated majestically on his glorious throne. Christian tradition interprets this as a vision of the last judgment, when the thoughts, words, and deeds of the entire human race are weighed in the balance by Jesus the judge.
The Son of Man’s first action is to assume the role of a shepherd who divides the sheep of his flock from the goats. It is often pointed out that Middle Eastern herdsmen normally allow their animals to graze together and that sheep and goats tend to be valued equally. This makes it difficult to say for sure why the judgment is depicted as a separation of sheep from goats. Nevertheless, it is clear from the outset that the sheep represent the saints, for they are placed at the Lord’s right, which in ancient cultures represented the good, fortunate, or honorable place (see 1 Kings 2:19; Ps 110:1), while the left represented the bad, unfortunate, or dishonorable.
The verdict of the king is that those on the right are blessed by the Lord and are the beneficiaries of his kingdom. These have shown themselves to be children of the Father and thus heirs of his heavenly estate (see 5:9, 44–45). Ever since the foundation of the world this plan of salvation was in place in the grand designs of the Almighty.
The reason for all this is then revealed. Whatever else can be said of the righteous, they have led lives of generosity and compassion toward others. They supplied basic human needs to the hungry and thirsty of the world. They took in a stranger, put clothes on the naked, sat at the bedside of the ill, and helped comfort those in prison. Serving their fellow human being through acts of kindness and mercy has secured their heavenly inheritance. The sheep are surprised to learn, however, that in caring for the needy they have cared for the Lord himself.