Then the king will say to those on his right, ‘Come, you who are blessed by my Father. Inherit the kingdom prepared for you from the foundation of the world. For I was hungry and you gave me food, I was thirsty and you gave me drink, a stranger and you welcomed me, naked and you clothed me, ill and you cared for me, in prison and you visited me.’ Then the righteous will answer him and say, ‘Lord, when did we see you hungry and feed you, or thirsty and give you drink? When did we see you a stranger and welcome you, or naked and clothe you? When did we see you ill or in prison, and visit you?’ And the king will say to them in reply, ‘Amen, I say to you, whatever you did for one of these least brothers of mine, you did for me.’

Matthew 25:34-40

Matthew 25: 35-46

All the various things listed in this passage (giving people food and drink, clothing them, visiting them) become works of Christian charity when the person doing them sees Christ in these “least” of his brethren.

Here we can see the seriousness of sins of omission. Failure to do something which one should do means leaving Christ unattended.

“We must learn to recognize Christ when he comes out to meet us in our brothers, the people around us. No human life is ever isolated. It is bound up with other lives. No man or woman is a single verse; we all make up one divine poem which God writes with the cooperation of our freedom.”

J. Escrivá
Christ Is Passing By, 111

We will be judged on the degree and quality of our love (cf. St. John of the Cross, Spiritual Sentences and Maxims, 57). Our Lord will ask us to account not only for the evil we have done, but also for the good we have omitted to do. We can see that sins of omission are a very serious matter and that the basis of love our neighbor is Christ’s presence in the least of our brothers and sisters.

St. Teresa of Avila writes:

“Here the Lord asks only two things of us: love for his Majesty and love for our neighbor. It is for these two virtues that we must strive, and if we attain them perfectly we are doing his will. The surest sign that we are keeping these two commandments is, I think, that we should really be loving our neighbor; for we cannot be sure if we are loving God, although we may have good reasons for believing that we are, but we can know quite well if we are loving our neighbor. And be certain that, the farther advanced you find you are in this, the greater the love you will have for God; for so dearly does his Majesty love us that he will reward our love for our neighbor by increasing the love which we bear to himself, and that in a thousand ways: this I cannot doubt.”

Interior Castle, V, 3

This parable clearly shows that Christianity cannot be reduced to a kind of agency for “doing good.” Service of our neighbor acquires supernatural value when it is done out of love for Christ, when we see Christ in the person in need. This is why St. Paul asserts that “if I give away all I have...but have not love, I gain nothing” (1 Cor 13:3). Any interpretation of Jesus’ teaching on the Last Judgment would be wide of the mark if it gave it a materialistic meaning or confused mere philanthropy with genuine Christian charity.

Matthew 25: 40-45

In describing the exigencies of Christian charity which gives meaning to “social aid,” the Second Vatican Council says: “Wishing to come down to topics that are practical and of some urgency, the Council lays stress on respect for the human person: everyone should look upon his neighbor (without any exception) as another self, bearing in mind above all his life and the means necessary for living it in a dignified way ‘lest he follow the example of the rich man who ignored Lazarus, the poor man’” (cf. Lk 16:18-31).

“Today there is an inescapable duty to make ourselves the neighbor of every man, no matter who he is, and if we meet him, to come to his aid in a positive way, whether he is an aged person abandoned by all, a foreign worker despised without reason, a refugee, an illegitimate child wrongly suffering for a sin he did not commit, or a starving human being who awakens our conscience by calling to mind the words of Christ: ‘As you did it to one of the least of these my brethren, you did it to me.’’’

Gaudium et spes, 27

Catechism

1970

The Law of the Gospel requires us to make the decisive choice between "the two ways" and to put into practice the words of the Lord. It is summed up in the Golden Rule, "Whatever you wish that men would do to you, do so to them; this is the law and the prophets." The entire Law of the Gospel is contained in the "new commandment" of Jesus, to love one another as he has loved us.