A word of warning. One thing that any Protestant knows is that there no such religion as “Protestantism”. Read carefully this paragraph from the document that @Dovekin quotes:
However, since these Churches and ecclesial Communities, on account of their different origins, and different teachings in matters of doctrine on the spiritual life, vary considerably not only with us, but also among themselves, the task of describing them at all adequately is extremely difficult; and we have no intention of making such an attempt here.
The Lutheran church is one thing, the Southern Baptist Church is a different thing, the Assemblies of God are something else again, and so on. If you start off by suggesting that it’s possible to “explain what Protestantism is” in a few sentences, you’ll be running the risk that your Protestant friend—if he has studied his faith at all—will pull the rug out from under your feet with no great difficulty.