Unfortunately, as a former Baptist (but still basically protestant), I knew a great many who were at basically an infancy level of spiritual maturity. Those who appeared to know more (such as many baptist preachers) were just spouting the dogma they were taught in seminary. The problem is, I know a great many Catholics who do the exact same thing!
I have seen a few from many denominations who really know their stuff and genuinely believe in certain doctrine and dogma. They have a little more than the standard dogmatic arguments, and some even genuinely want to see the truth.
Believe it or not, most people don’t intentionally spread lies. To lie is to intentionally tell something that isn’t truth as though it were. For example, I vehemently disagree with Catholics on a variety of issues. As far as I know, every bit that I believe is the truth. I don’t believe in the authority of the Catholic church, etc. I’ve studied this stuff a lot, and certainly believe that I’m only sharing the truth as God has given it to me.
Likewise, I have good Catholic friends, and I’ve seen some of you who debate here, who genuinely believe that Catholic teachings are correct. Whether they are or not, some of you do believe this, and so when you share that information, you aren’t spreading lies – you’re simply misguided in what the truth is.
I believe that truth is objective (not relative, not subjective). Thus, my constant prayer is that God will continue to open my mind more and more, and show me his truth more completely, so that I can better understand it. Likewise, for my friends, and in general for those I debate with, I pray that they too will have their minds opened, so that they can come into a better understanding of the truth of God.
In other words, it is quite possible to be genuine in your beliefs (not deceptive), and yet still be wrong. This is simple necessity – there are protestants and Catholics who both believe genuinely, but not both can be right.
Don’t pity protestants, nor be angry with them. There are enthusiastic ones that will get under your skin, because they don’t explain how they came to the understanding they came to. We all have certain pre-conceived notions based on the ideals we were raised with as young children, and these notions make us interpret the facts differently. For some, this leads to atheism. For others, it’s Catholocism, Protestantism, or something else.
The best response you can give is to pray. And don’t just pray that “the eyes of the sinful, ignorant protestants will be opened so they’ll see that your true Church is right”. Instead, pray that those who seek him will find him, that his truth would be made clear to those who seek, and that the minds of all would be opened more fully.
Pray for yourself in this regard as well – remember, you’re human. This means you can make mistakes (just like Protestants can), and this means that the teachings and beliefs that you accept could be wrong (just like Protestants could be wrong).
Most people don’t intentionally spread lies. The problem is that there isn’t a place where God (himself) has written out every detail of truth in a way that we can all understand it.