Really, all he may have done is put forth a declaration making it the state religion of the Roman Empire. This presupposes that there was already a significant number of Christians already in the Empire. This conversion was the the doing of the Apostles & Evangelists, not of Theodosius.
The world was already becoming Christian, I would argue that those like Constantine and Theodosius certainly helped the spread of Christianity. But, they weren't the main force behind it. The Church (and the Lord) are to credit for the spread of Christianity, not individuals.
Converted the american Indians to catholicism
That isn't something I wouldn't exactly be proud of... Christopher Columbus isn't exactly celebrated by everyone in South America. I used to love history of groups like the Aztecs and others, and I would argue that the arrival of westerners wasn't exactly good, especially when the westerners tried to convert the Native Americans. When the Native Americans refused to convert, we all know what happened to them.
I'm personally see the Russians as more of an example to follow. When they arrived in Alaska, some of their explorers and traders definitely mistreated and harmed the Natives, but when the missionaries arrived, they peacefully converted the Natives (who had been prepared to receive Orthodoxy through their own theology) to Orthodoxy.
However... We could infinitely debate and argue the controversies of how Spain, Britain and France treated the Natives. We could point fingers all day and play the blame card to the end of time. However, I'm simply trying to point out that the West isn't so innocent when it came to converting others to Roman Catholicism.
Christianus, It's your right to support your Church, if you want to be Catholic because of your love for Latin America/Spain, then that is certainly your choice. However, even in your love for the Catholic Church & Spain, you are going to have to answer to other people on the way that country and that Church treated groups that didn't agree with them or that refused to convert.