And all who believed were together and had all things in common. And they were selling their possessions and belongings and distributing the proceeds to all, as any had need. Acts 2:44-45. For I do not mean that others should be eased and you burdened, but that as a matter of fairness your abundance at the present time should supply their need, so that their abundance may supply your need, that there may be fairness. As it is written, “Whoever gathered much had nothing left over, and whoever gathered little had no lack.”
2 Corinthians 8:13-15
Wonder why great Christians like Reagan and most evangelicals today are ardent capitalists? It's because the lord believes in fairness and opportunity where as socialism encourages the state to have absolute control over everything and let it steal from the people. Wonder why socialist countries execute all christian? It's because Marx said "religion is the opiate of the masses" socialists think religion is a drug and given why we have a war on drugs you can imagine what they think of religion.
In the Book of Acts, early Christians actually practiced such. By lying about giving everything to the church, Ananias & Sapphira were struck dead by God. That's not just socialism - that's soviet communist. Some point to scriptures about not belonging to secret societies, but most labour unions are not secret societies (though leadership negotiations can seem such). Its no more a sin than belonging to a country.
If this person is asserting that socialism is a sin, he needs to prove that:
a) socialism is something that a person can do (sins are crimes done by people against deities - I doubt that a person can 'do' socialism - it's a government system, not an act)
b) some religion somewhere in the world condemns the act of socialism as sinful, and
c) that the religion that condemns socialism is correct (after all, if the god that condems socialism doesn't exist, calling it sinful doesn't make sense - you can't commit a crime against a non-existent entity).
Now, I'm opposed to the concept of sin in general, as it is a religious concept. I only believe in immorality. Nevertheless, I won't do the kritiks and instead focus on whether socialism is a sin.
If socialism were a sin, it must have violated some kind of religious tenet. I am in complete agreement with the post above me. Do redistribution or putting modes of production in the hands of workers violate any of those? Uhh... No. The only way you can even come close is to say progressive tax is theft or something, but even that's a stretch and a semantics question.
France is pretty socialist considering their labour rights, but you don't see people saying Hollande is sinful for implementing such policies (although he did commit sins not related to socialism).
Of the 7 deadly sins, none of them include: "Distributing wealth" or "Putting the modes of production in the hands of workers". Instead, it actually contradicts the deadly sin of Gluttony or of "habitual greed". In a socialist system, the wealth is distributed thus permitting that no one goes hungry and that the rich, ultimately, help the poor the most (In a perfect system).