"It doesn't matter if someone is Protestant or Catholic or Pentecostal or whatever; God looks at the heart. Even if somebody believes wrong things, that doesn't mean they're not a Christian. What matters is that you're sincere and your heart is right with Jesus."
(And, no, I don't mean shut the discussion down with one- or two-line statements like "how stupid" or "that's just heresy". I'd like to hear how people would refute such a statement above by the use of Holy Scripture, the Fathers, etc.)