ESV - 40 And the King will answer them, 'Truly, I say to you, as you did it to one of the least of these my brothers, you did it to me.
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If we were to go a step further , it means the least of these are our brothers and sisters that are believing Jews these that are persecuted in Israel today, that is why Yeshua said unto me.
To me, "the least of these" means of this group whoever you think of as the least deserving of your help. And the key is whoever you think, not Jesus. To Jesus they all need help.
Maybe to you it might be the biggest sinner. Maybe the least socially acceptable. Maybe the one who won't give you as big of feeling of self righteousness for helping them. Maybe the poorest or even the richest. Your perspective is everything because they are all equal to Jesus.
When you give or show kindness to those you consider "the least of these" Jesus is pleased and of course He is most pleased when you get to where like Him, they are all the same and all deserve your help and kindness.
Jesus had a specific group in mind. It isn't "left up to you" to decide who these "least" are. He was talking to people who thought much too much of themselves. They knew who he meant.
If He said the same thing today to a congregation the people would know who He would be referring to. It is the people who race through your mind when "the least of these" hits your ears. That's who He's referring to when He says it to whomever. Everybody knows who "the least of these" are to them.