Aargh... I might just change my country to the Netherlands for the achievement and start all over again.
Sigh. I guess I reformed too late, and I had no idea that prestige is the factor that keeps nearby centres of reformation from actually reforming your provinces. However,once I switched over to reformed, I not only found that I was too late to get my very own centre, but also the two neighbouring centres of reformation, both protestant, started converting my provinces like crazy. So now I'm barely above 76% Religious unity, and won't be able to convert other provinces for decades. And when I decided to start the colonizing game, I found that Portugal had pretty much taken over all of the coast of South America I could actually see...
In short, I fail. Should I take Exploration ideas second in order to be able to secure a foothold? And if I decide to switch religion, should I wait and see what religion is close, or should I react immediately and get a Center of Reformation myself? Also, I pretty much conquered every OPM I went to war against and annexed their provinces on the spot. Should I have opted for vassaldom instead?