Unbelievers because they didn't do as they were expected to do....the works.
Otherwise why wasn't the parable about unbelievers period? Why bother with all the going on about feeding the poor? While instead it was about the details that rendered them not true beleivers, as they clearly thought they were believers.
Let's simplify this... can you make it to heaven with no works/not taking care of the poor?
In the Parable of the Sheep and the Goats, we are looking at man redeemed and saved, and man condemned and lost. A casual reading seems to suggest that salvation is the result of good works. The “sheep” acted charitably, giving food, drink, and clothing to "the least of these". The “goats” showed no charity. This seems to result in salvation for the sheep and damnation for the goats.
However, Scripture does not contradict itself, and the Bible clearly and repeatedly teaches that salvation is by faith through the grace of God and not by our good works (see John 1:12; Acts 15:11; Romans 3:22-24; Romans 4:4-8; Romans 7:24-25; Romans 8:12; Galatians 3:6-9 & Ephesians 2:8-10). In fact, Jesus Himself makes it clear in the parable that the salvation of the “sheep” is not based on their works—their inheritance was theirs “since the creation of the world” (Matthew 25:34), long before they could ever do any good works!
And who are “the least of these” anyway? Many assume that they are the poor and marginalized people of the world. But are those who Jesus is really talking about? Matthew 25 gives few clues as to who “the least of these” are. They’re described only as hungry, thirsty, homeless, naked, sick, and imprisoned.
A common practice of biblical interpretation is this: If we don’t understand something in a specific passage, we need to study the surrounding text—whether it be the immediate chapter or the larger book. So who else in Matthew went hungry, thirsty, homeless, naked, sick, and imprisoned? We find such a group in Matthew 10:1–42, where Jesus sends out the 12 disciples to preach about the kingdom.
Parallels abound between Jesus’ words in Matthew 10 and the description of “the least of these” in chapter 25. In chapter 10, the disciples had no money, bag for food, or drink (vv. 9–10; compare to the hungry and thirsty in ch. 25). They had no extra clothing (v. 10; the naked in ch. 25), and they had no home to stay in (vv. 11–14; the strangers in ch. 25). Jesus said they would often be arrested (vv. 17–20; the prisoners in ch. 25). Even the order of these circumstances is a near match. Also recurring is the idea that one’s response to Jesus’ representatives is a response to Jesus himself: “Whoever receives you receives me, and whoever receives me receives him who sent me” (10:40). And the rewards language in chapter 10 is conspicuously similar, “And if anyone gives even a cup of cold water to one of these little ones because he is my disciple, I tell you the truth, he will certainly not lose his reward” (v. 42).
The parallels between the two passages are no accident and make a strong case that the same group is in mind. The “least of these my brothers” are the disciples, followers of Jesus who carry his message. Jesus’ “brothers” in the Gospel of Matthew are always his disciples (12:48–50; 28:10). That specific language is used of no one else.
This understanding not only has biblical warrant, but also avoids placing unseemly tension between Jesus and Paul regarding salvation. If “the least of these” are the poor and marginalized people of the world , then according to Jesus, individual salvation depends on works. It’s difficult, therefore, to reconcile those readings of Matthew 25 with Paul’s teaching: that we are saved by God's grace, through faith, not by anything we do (Ephesians 2:8–9).
However, if “the least of these” are Jesus’ messengers, then one’s salvation is dependent upon his or her response to the proclamation of the gospel, which accords with Paul’s teaching. It is, therefore, ultimately a question of their relationship to Jesus himself.” We don’t have to be terrified that our salvation is at risk if we pass by a poor person on the street. And traveling full circle, while Scripture doesn’t identify “the least of these” as the poor and needy, in no way does it diminish the biblical mandate to care for the marginalized and underprivileged. Our actions matter, but they do not contribute anything to our salvation.