"Then He will also say to those on the left hand, 'Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels: for I was hungry and you gave Me no food; I was thirsty and you gave Me no drink; I was a stranger and you did not take Me in, naked and you did not clothe Me, sick and in prison and you did not visit Me.' Then they also will answer Him, saying, 'Lord when did we see You hungry or thirsty or a stranger or naked or sick or in prison, and did not minister to You? Then He will answer them, saying, 'Assuredly, I say to you, inasmuch as you did not do it to one of the least of these, you did not do it to Me.' And all these will go away into everlasting punishment, but the righteous into eternal life."
"Depart from Me, you cursed, into the everlasting fire prepared for the devil and his angels" - The two places that God has prepared for eternity stand is sharp contrast with one another. God prepared heaven, the kingdom of absolute bliss, and hell, the realm of absolute agony. God intended for man to be with Him in heaven, and He intended for the devil and his angels to suffer the eternal anguish of hellfire. However, God will allow man to share in Satan's punishment, should he choose such for himself. The fact that wicked men will suffer the punishment of the devil certainly shows God's intolerance of sin.
These will be asked to depart from God's presence because of (i.e., "for") their sins of neglecting to feed the hungry, neglecting to clothe those who are lacking sufficient covering, and neglecting to minister to the sick and imprisoned (cf. James 4:17). Regardless of what other sins may or may not have been committed, God evidently views these sins of neglect as serious enough to send one to the same fate as the devil and his angels. To neglect the needy, especially fellow disciples, proves the absence of faith and love that are essential to genuine, Christian character.
In Matthew 25:44, the wicked ask when did they ever see Jesus in such a condition and fail to help Him as He has stated. Certainly if they had recognized Jesus in such poor circumstances, they would have helped Him, knowing that such would lead to blessings for themselves. A lesson that can be seen here is that the attitude that is most pleasing to God is the one that shows compassion and good will toward the needy, without consideration of whether or not such will lead to blessings for oneself.
We should desire to live benevolently simply because it is the right thing to do. Fear of punishment is a motivator for many to do right, but one should grow to the point where love is his motivation and not fear (cf. I John 4:18).
To neglect or abuse a follower of Jesus is to neglect or abuse Him (e.g., Acts 9:4). "And these will go away into everlasting punishment, but the righteous into eternal life" - Although different English words are used here (i.e., "everlasting" and "eternal"), the original Greek word is the same, and the meaning is the same in both cases. The doom of the wicked will last just as long as the joy of the righteous; that is, forever. The Greek word for "punishment" communicates the idea of misery and suffering that is purposely inflicted.
Some have tried to twist Matthew 25:46 into teaching the false doctrine of annihilation (i.e., the idea that one will not suffer forever in hell; rather, he will cease to exist after being tormented for a certain length of time; cf. 10:28). Such an idea cannot be substantiated from the New Testament.