Altruism is a concept of another strain of atheistic materialism: namely, Comtean positivism.
But Christianity does indeed demand boundless self-sacrifice.
"But I say unto you, That ye resist not evil: but whosoever shall smite thee on thy right cheek, turn to him the other also.
And if any man will sue thee at the law, and take away thy coat, let him have thy cloak also.
And whosoever shall compel thee to go a mile, go with him twain.
Give to him that asketh thee, and from him that would borrow of thee turn not thou away.
Ye have heard that it hath been said, Thou shalt love thy neighbour, and hate thine enemy.
But I say unto you, Love your enemies, bless them that curse you, do good to them that hate you, and pray for them which despitefully use you, and persecute you;
That ye may be the children of your Father which is in heaven: for he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.
For if ye love them which love you, what reward have ye? do not even the publicans the same?
And if ye salute your brethren only, what do ye more than others? do not even the publicans so?
Be ye therefore perfect, even as your Father which is in heaven is perfect." - Matthew 5:39-48.
The Cross is not God saying “let’s give it our best shot and hope some sinners out there get the hint” — but a promise. The giving itself is intended to be a picture of God’s purposeful sacrifice.
The Cross is not a hint, or a promise or a picture. It is at once the entire message and the its fulfillment.
"And he said to them all, If any man will come after me, let him deny himself, and take up his cross daily, and follow me." Luke 9:23.
A wise one does not keep on shoveling pearls at pigs, to use another biblical maxim.
The pearls are a metaphor for the holy truths that He revealed - not an instruction to avoid making personal sacrifices for His enemies or ours.
The Father sent, and Jesus went, for the sake of all who would believe. Without getting into Arminian/Calvinist debate on specifics, God can easily link causation in a direction which would seem “backward” to us time dwellers.
No sacrifice that man can carry out can equal the cross. It can only feebly depict it at best. And these have to be purposeful, wisely targeted sacrifices to be truly Christian giving. Christendom could easily erase itself from the earth if it pushed “sacrifice to your enemies” to a mindless maximum.