Medieval polemic distinguished actual belief from what Muslims merely profess

Evidence

While there was no question about what Islam believed itself to believe, there was some attack on the Islamic belief itself. Thus Peter the Venerable, in recognising the profession of faith, the shahadah, wished to distinguish between what Muslims believed and what they thought they believed. ‘I know that this is what you understand and what you profess: that you truly believe in God and the true God. But whether this is true a later and unanswerable reason will show you, also by the Spirit of God.’ — Norman Daniel, Islam and the West, 253