Mat 25:40 And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me.
If you belong to the school of thought that these "brethren" are any destitute persons whatsoever, then this passage will tend look like a general rule for doing physical works of mercy, and certainly in a spirit of Christian love nothing prohibits that. No law against doing good things for people.
But the other school of thought here is that these are truly Christ's brothers as he has elsewhere defined them, those who do His Father's will (Matthew 12:50), the principle act of which is simply believing in Jesus His Son (John 6:27). Thus believers are the "brethren," and not just everyone.
This latter position fits the context well, the judgment at the end of the world, after all the great trials that come on the earth. These "sheep" appear to be people who lived through all that, the survivors. During that horrific time, the persecution of believers will be worse than at any other time in history, and many who are not formally Christian will be put in a position to help or not help persecuted Christians, whose suffering will take all the forms described in this passage and more.
So is it possible that this passage is not meant to give merciful Christians an early pat on the back, but to demonstrate the great mercy of King Jesus, Who at His coming will extend His grace not just to those who endured persecution for His name's sake, but also those who were willing to risk their own well-being to give aid to those true brethren of Christ, those who believe in Him?
Qualifiers are a GOOD thing.
Too bad so many ignore them...
I can see what you are saying here and can somewhat agree that brethren would be/is those who belong to Christ.
I can also see that there may be those who do these things without doing them as one of the brethren.
But, I believe that Christ teaches us how to believe and how to love and this passage is meant to explain that it is not the external works of the law, but acts of mercy/kindness/compassion and selflessness that bear witness to our love for Christ and for our neighbor.
Remember in the story of the good Samaritan, the neighbor was not who His listeners expected but one who others would not have expected to give help. We do not always know who are brethren/neighbors. Love can come to us from unexpected people and we should love those who do not expect it from us.