"Are there any socialist countries that have maintained robust free speech rights and freedom of religion?"
Canada. My country, the UK. Much of Europe, especially the Scandinavian countries: Sweden. Denmark. Finland. Iceland. Norway was effectively the only country that emerged out of this recession unscathed. Not bad for a Social Democracy, eh?.
"I can't think of any."
If you wanted to maintain some semblance of credibility, you should have stopped after 'I can't think.*' [/Spartan Laconic Wit]
"It seems the trend of all socialist countries is to curb speech and/or religion to some extent."
How's that Patriot Act doing for ya over there eh, Andy? Y'know, that introduced by your God Emperor of Fucking Up, George Dumbya Bush? Oh, and I hear a lot of your Religious Right, Neocunt & Teabagger ilk wanting to curb not only speech but the right to not believe; and not just speech, but the human rights as a whole, of anyone not exactly like you: 'Liberals', Gays, Women wanting abortions, Atheists, Pagans/Wiccans et al.
"China is an extreme example"
Only with Falun Gong, as far as I can tell. There's still Buddhists worshipping and going on pilgrimages in Tibet. There's still Christian churches in China, both Catholic & Protestant (two words: Watchman Nee). I even hear that there's a $cientology Org or three over there.
"while what you see in England is the start of a slippery slope."
And you're an expert of UK social affairs eh, Andy? Lived here for years, have ya? Don't exactly keep up on current events, do ya pal? Speaking as a Brit, we have a state religion (Church of England). We've had women vicars etc in such for years now; gay male clergy too. It's still one of the richest organisations of it's kind in the world (certainly one of the biggest landowners in the UK, next to the Queen & the MoD. Own a helluva lot of property too). They're still here. Pop quiz, hotshot: guess who they have buried in Westminster Abbey, the central place of worship of the Church of England (and where almost all of our royalty are buried)? Charles Darwin. He's one of only five non-royalty persons buried there. Because of what he discovered - and thus the doctrinally lethal implications such would have/has on not just their, but all Bible-based Christianity as a whole - do you seriously think they would have done so? PROTIP: When "On the Origin of Species" was first published, it was such a massive hit that it sold more than the Bible at the time. Yet, the C-of-E wasn't worried about such in the slightest. Indeed, it embraced what it represented: a total paradigm shift in how people would think about humanity's origins.
And why is that, do you ask? Why Darwin's ToE - with it's Genesis-destroying implications - was accepted by the C-of-E with little or no complaint? Simple. Because the Church of England - even to this day - openly admit that much of the Bible is purely fable and metaphor. As in not to be taken literally.
The Church of England. Originated in King Henry VIII's time. Still exists in the 21st Century. Destroyed argument much, Andy? 'Slippery Slope'? About as likely as finding one in Holland, pal. (and look how hyper-tolerant that country is, even though is, in terms of a majority religion, almost purely a Protestant country - William of Orange, and all that jazz; Amsterdam's cannabis cafes, brothels & sex shops, thriving and open LGBT community, yet one of the lowest teenage pregnancy rates in Europe. Go figure.)
"So I wonder, why would any thinking Christian ever support socialism? It is beyond me."
Hmmm, I wonder that too. You could always ask... y'know... that bloke who fed 5000 people with just five loaves & two fishes. For free. How he healed the sick, lame, blind - and even brought one back from the dead (no defibrillators/adrenaline injections around then, natch). And never asked for a penny in return for his medical services (UK's National Health Service, anyone?!). How he intervened to save a woman from the death penalty, interceding in her case, pleading clemency from her executors and those who'd ordered such for her 'crime'. Did he send her a lawyer's/solicitor's bill? (Public Defender [US], Legal Aid [UK], anyone?!).
So I guess the charities in your locale have yet to see a sudden influx of donations, nor has a homeless family been given your home to live in, and as a result, you're now working full-time for, say, Christian Aid or similar organisation eh, Andy? Remember this little titbit of Scripture?:
Matthew 19:21 (KJV): 'Jesus said unto him, If thou wilt be perfect, go [and] sell that thou hast, and give to the poor, and thou shalt have treasure in heaven: and come [and] follow me.'
Therefore I wonder...
Why would any thinking* right-wing fundie, who dares to call themselves a Christian like you, not do as their Lord and Saviour commanded them, as their own Scripture orders them to? It is beyond me.
Moral: To reiterate what I said above, 'If you wanted to maintain some semblance of credibility, you should have stopped after 'I can't think.*'; there's an old saying that's pertinent in your case (emphasis added):
'It is better to be thought a fool and remain silent, than to open your mouth and remove all doubt.'
Words to live by. Pity that you and the rest of your fundie ilk can't. Four Words, Andy: Quakers. Unitarian Universalists. Amish.
*- If they even have the capacity to think, that is.