When the Son of Man comes in his glory, and all the holy angels with him, then he will sit on the throne of his glory. All the nations will be gathered before him, and he will separate them one from another, as a shepherd separates the sheep from the goats. He will set the sheep on his right hand, but the goats on the left. Then the King will tell those on his right hand, ‘Come, blessed of my Father, inherit the Kingdom prepared for you from the foundation of the world; for I was hungry, and you gave me something to eat. I was thirsty, and you gave me drink. I was a stranger, and you took me in. I was naked, and you clothed me. I was sick, and you visited me. I was in prison, and you came to me.’
Then the righteous will answer him, saying, ‘Lord, when did we see you hungry, and feed you; or thirsty, and give you a drink? When did we see you as a stranger, and take you in; or naked, and clothe you? When did we see you sick, or in prison, and visit you?’
The King will answer them, ‘Truly I tell you, because you did it to one of the least of these brothers of mine, you did it to me.’
Then he will say also to those on the left hand, ‘Depart from me, you cursed, into the eternal fire which is prepared for the devil and his angels; for I was hungry, and you didn’t give me food to eat; I was thirsty, and you gave me no drink; I was a stranger, and you didn’t take me in; naked, and you didn’t clothe me; sick, and in prison, and you didn’t visit me.’
Then they will also answer, saying, ‘Lord, when did we see you hungry, or thirsty, or a stranger, or naked, or sick, or in prison, and didn’t help you?’
Then he will answer them, saying, ‘Truly I tell you, because you didn’t do it to one of the least of these, you didn’t do it to me.’ These will go away into eternal punishment, but the righteous into eternal life. (Matthew 25:31-46)
The teaching is striking and memorable in several different ways. First, there is the clear, plain rightness of the actions called for. Rarely do you find a teaching that is so plainly and thoroughly good. Here is a teaching that can change the world -- and to the extent that people listen and follow, it does in fact change the world. These are the words that created Mother Theresa of Calcutta and brought thousands of people to help her. The same words have created followers for Jesus in every age.
Next is the profound fairness of the way the groups are separated. Earlier in his teachings Jesus had said, "With the measure you use, it will be measured to you," and "Blessed are the merciful, for they will be shown mercy." Each person is shown to choose how the Judge will treat him by how he treats others. Jesus' teaching silences the complaint “God is unfair”. Can a person complain it is unfair for the Lord to treat them in the same way they treat others? ("You can't treat me the way I treat everyone else! It's unfair!" -- That person has just testified against himself, that he treats others badly, treats them in a way he would not want to be treated.) There is an unanswerable justice to it all.
There is the tender kindness shown to "the least of these brothers of mine" -- to show that the Lord sees himself in each of them. There is no room for doubt about whether the judge of all the earth has compassion on everyone, even the least. The only topic on the table is whether we have the same compassion.
The focus of Jesus' teaching is not on the judge, but on justice, on compassion, on mercy, on the least of the brothers -- so it's easy to miss what is not the focus: Jesus portrays himself as the one judging the world. He doesn't make a big deal out of his status; that seems taken for granted and not his main point. But it might explain how he knows so clearly what the Last Day will be like.