Where do you get that idea?
The teaching of the Church is obviously that being a visible member of the Church is best. What church doesn't think that about itself?
But Church teaching has always allowed for the idea of "invincible ignorance": that one will not be held accountable for that which he is unable to know.
With God, all things are possible, after all.
While I have my disagreements with aspects of Protestant theology, I hope that I may nevertheless approach discussion thereof with charity. I certainly try to do so with my dad, who is Methodist, as does he with me.
I think it was the Council of Trent or Nicea or something like that. Maybe Vatican II. Anyway, the gist of it was that anyone who says salvation comes by faith alone is anathema.