It's true that Estonia was originally Christianized by a crusade, and there was an "ancient independence war" in the 13th century. However, many Estonians conveniently forget that slavery was already practiced in pre-Christian Estonia. Serfdom did eventually come into effect as well; the liberties of native Estonians rapidly declined after the significant Jüriöö uprising in 1343-1345.

However, it's not right to say that nothing good came out of Christianity in Estonia. Most importantly, it was thanks to Lutheran priests and church education that Estonia had a high amount literate serfs compared to the rest of the Russian Empire. Those serfs became the basis of an Estonian intellectual class, leading to the national awakening movement in the mid-19th century. The movement in turn led to the creation of Estonia as a nation state.

edit: OHMG I didn't notice how long I managed to babble. :/

As an Estonian this is what I was thought.

Basically pre-Christianity Estonian society lived as different counties. Think of Greek city states but on a much smaller scale. If there ever was a bigger threat local leaders of each county came together and discussed uniting forces.

Before 13th century there was a lot of raiding to be done by boats (we even had our local vikings who, if certain ancient texts are to be trusted, managed to capture a Norwegian viking king once) and by land to the local Latgals or Vene people, or fight among ourselves.

Christianity was spread by a crude but simple "join or die" mechanic at first, well not always death but paganism was the ancient equivalent of radical Islam, more so as Estonians showed resistance against them.

Later basically your rights became mighty small if you weren't a Christian so most people were reasonable and accepted a religion if it made their life that much easier. Basically like Muslims "allow religious freedom" only problem is that those of different religion have to pay actual money ( or gold in ancient times ) to keep that right.

But the basics was simple, invade with steel and sword, conquer and kill local "radical pagans" who resist and start nudging the local peasants towards Christianity. Once you rule by force its much easier to sway the population. Pretty similar concept of the crusades to the holy land, those who fought, kill. Those who don't, start telling them of all the glory Christ brings them.

The "oppression" came because it was the first time the land of Estonia came to serve a foreign rule. Before we raided and enslaved for our own lands. Now we were the ones being enslaved.

Also every Estonian gets thought that we even managed to win one battle against the ancient German tanks, that were the steel dressed men. Sadly as told by ancient chronicles, we lost most of the battles. Even our local "king"/"William Wallace" figure got killed.

So we simply were sitting on our asses for too long and didn't expect some steel train of pain to roll through and we weren't united enough to really fight the Germans. Experienced raiders as the old Estonian people were, too many well equipped Germans to face.

The Jüriöö ülestõus is a "great" historical event as it caused a big enough chain of events and gave us important information. Firstly that the local population, Estonians, weren't happy at all and wanted the old ways back of their own free rule. Secondly that enough people were brave enough to even attempt such a thing under a common goal of freeing the lands, so a certain "united people" front feeling had to be there.

We even had our own "Red wedding" (GoT spoiler) for our local resistance leaders who were invited to discuss peace only to be killed on the spot. Kind of a dick move to pull off and think of yourself as a "holy order".

Also Denmark was afraid to keep such a rebellious land and sold us to the German knights order who more or less cut off locals from any kind of self management being afraid of another uprising.

The revolution actually lasted a full 2 years as the Germans/Danes struggled to calm it down.

So the wording would be more like "it brought oppression and slavery to Estonians" and not simply to Estonia. As for the effect? Our society as such ceased to exist, we were the serfs of Germans. Kind of like Americans conquered native American people. Our only well preserved records are from German and later Livonian or "Western" influenced chronicles who noted the events down that happened during the crusades (and beyond). As someone already said we only managed to truly awake as a nationality or nation in the 19-20th century. We were a very fractured society, well we still had our language and communicated among ourselves even pre-13th century, there is that.

So since the 14th century to the 19th locals slowly gained more and more rights as the local upper class mingled with the people of the land. So it took about 500-600 years off farming and slowly getting more educated, more back into ruling our own land to reach for our own nation. At least in our independence war Denmark was a big help this time around and gave us support to defeat the Red army.

In the long run the effects were similar to what other Western nations had gained through Christianity, a good basis to spread basic education ideas like reading, writing and overall moral education. So in the long run the Estonians went from earth worshippers (as the Germans were themselves in the times of the Roman empire) to Westernized people.

The "what if" question is always interesting though. What if Estonia hadn't been conquered? I would think a similar scenario to the Native people of the Americas would of occurred. We more or less would of been happy enough in our ignorance and advanced at our own pace.

Our cultural/lingual neighbors Fins are a good comparison though. They also shared a similar history of being crusaded and taken control over, only in their case instead of Germans it was the Swedes.

So historically it was simply "that time". That right time were some cultures were advanced enough and big enough to start expanding their influence outwards and it was easiest to be done by sword.

As was the case European culture started spreading from the Mediterranean area and outwards so once Germany gained enough savvy and unification from Roman ideas, it spread to Scandinavia and from there to Finland, Baltics. By that time Russia itself was advanced enough on their side of the land that their culture grew strong to oppose the one coming from the West and behold, we are in the 21st century and still struggling with the same baggage of trying to enforce our ideas on other regions.

So yes your friend is right, it brought slavery/oppression but as we have one string of history for everyone, it also obviously brought us forward to the 21st century. I mean the concept is the same everywhere, this is how it was done. Even Germany unified their regions by battling each other, Rus people unified as they battled among each other.

You may want to post over at /r/Christianity as well.