I am aware of two interpretations of the expression, “the least of these my brethren.” It is used in the parable of the sheep and the goats in Matthew 25:31-46.
One view emphasizes “brethren” and indicates that we will be evaluated by how we receive the servants of Christ. This interpretation notes that receiving the messengers of Jesus brought a blessing when the 12 were sent out in mission to the towns of Judea (Matthew 10:1-16).
Another passage on that theme is Matthew 18, where there is a blessing to those who humble themselves like a child to receive the kingdom. (Mt. 18:1-4)
Both of those passages indicate that a faith response is what makes someone one of Jesus “brethren.”
The other view puts the emphasis on “the least of these.” This passage has been seen as a central motivator in missions of compassion to the poor. Some go so far as to say that since God has a preference for the poor, they are already his brethren, and so this is very inclusive. There is no need for the recipient of acts of mercy to have a faith response because they are already God’s children.
This interpretation stands behind a lot of Christian ministries of compassion and justice.
Now, how can we answer this from Matthew?
First, we have to include both parts of “the least of these” and “my brethren.” So Jesus is indicating something about social insignificance and about a faith response. I say that because of verses like “blessed are the poor in spirit, for theirs is the kingdom of God.” a recognition of need is a prerequisite to entrance into the Kingdom.(Mt 5:3)
Secondly, we notice in Chapter 25, there is a division between those who receive grace and are included and those who do not. In the parable of the 10 Bridesmaids, 5 are prepared and enter and 5 are left out. In the parable of the Talents, the third servant is cast outside for his lack of faith in his master. In the Sheep and the Goats, the nations are divided between those who are Sheep (who enter the Kingdom) and the Goats (those sent to eternal punishment.) So Matthew 25 does not support the idea of total inclusion of all people into the Kingdom of God. Some are outside, and can hardly be considered to be brethren of Christ.
Third, there is an equivalency between those who follow Jesus and becoming the least. The Sermon on he Mount as a manual for discipleship is opposed to self-sufficiency. The call to discipleship is the call to leave everything, (Mt:16:24)
The mission of the 12 involved self-denial and dependency on reception by those who hear the message. (Mt 10:8ff).
Finally, there is a connection between receiving Jesus messengers and receiving Jesus. Matthew 10:40-42 reads as follows.
40 “Whoever receives you receives me, and whoever receives me receives him who sent me. 41 The one who receives a prophet because he is a prophet will receive a prophet’s reward, and the one who receives a righteous person because he is a righteous person will receive a righteous person’s reward.42 And whoever gives one of these little ones even a cup of cold water because he is a disciple, truly, I say to you, he will by no means lose his reward.”
Notice the strong connection between giving water to “little ones” with their being a “disciple”.
I believe we do have a strong calling to generalized compassion ministries. We can find that all through the scriptures from the Law, the Prophets, the Wisdom literature, the teaching of Jesus and the Apostles. But that does not seem to be the meaning of this text.
This text does call us to compassion. Let us help the needy and seed justice for the poor.
But we can not separate sharing the compassion of Jesus from receiving the message about Jesus.