Let me explain it to you. Please, I’m going to be blunt so if this comes off as harsh or critical of Catholicism it is not. I’m just laying everything on the table and not beating around the bush.
First, you are not speaking about all Protestants (as you rightly point out). You are speaking about the “born again” types, also known as evangelicals. I’ll call them “born-again Christians” for short.
Second, what evangelicals believe is that a conscience conversion experience is necessary. This is called “being born again” or “getting saved.” Beliefs vary, but the simple idea is that after this conversion experience (if it was authentic) then the person has been born again and is a “new creation.” Fruits and evidence of this new birth should follow.
Third, “born-again Christians” believe that only those persons who can testify to having placed their faith in Christ are members of the church. You are not a Christian if you do not have faith in Christ.
Fourth, this means that, from the perspective of “born-again Christians”, those persons who:
have been baptized (at any age) without having put their faith in Christ
or who had their name listed on a church membership roll without having put their faith in Christ
or who made an inauthentic confession of faith (meaning they said they put their faith in Christ but they really never did)
or perhaps at one time they did have faith in Christ but later lost it and are now in a backslidden state
are not Christians. Either they never were saved or they lost their faith and apostatized.
This fourth point applies to any and all Christians. It applies to Catholics and it applies to other “born-again Christians.” “Born-again Christians” are strong believers that there are many unregenerate people in the churches. Everyone who is a member of the church is not saved. Your status in any particular church means nothing. You could be a the pastor, deacon, or Sunday school teacher and still be unregenerate. It doesn’t matter. Man looks at the outward appearance. God looks at the heart.
For many “born-again Christians”, the Catholic Church is one of those churches that places more emphasis on the outward things. They see the Catholic Church as believing things like: “if you’ve been baptized, you’re a Christian whether you have faith in Christ or not.” “Born-again Christians” would never lump all the members of their churches into the “Christian” category. They always recognize that while people may affect a Christian persona, they may not be truly regenerate.