Medieval Christians doubted Islam’s truth yet marveled at its success

Evidence

It was usually realised that Muhammad claimed to have been sent by God, and that there must have been something remarkable about his inspiration, to explain the otherwise incredible credulity of the Muslims. At the most fabulous end of the spectrum the explanation was said to lie in the fabrication of false miracles; at the other extreme, it was recognised that traditional Christian attitudes must take the Muslim version into account. — Norman Daniel, Islam and the West, 168