Abu Isa al-Warraq was a 9th-century Arab skeptic (born circa 815 CE) and an early critic of Islam and of religion in general. He was a mentor and friend to Ibn al-Rawandi and appears in his book "The Book of the Emerald" which is a bold attack on prophets and prophecy, Muhammad in particular, and the validity of the Koran.
al-Warraq was obviously a man of unwavering rationality and courage. He argued that God was unlikely to exist because, "He who orders his slave to do things that he knows him to be incapable of doing, then punishes him, is a fool."
Echoing European Enlightenment thinkers of the 18th century he also argued that we do not require revelations from God to work out that forgiveness is good; we should not heed the claims of self-appointed prophets if what is claimed runs contrary to good sense and reason. Contemporary skeptics such as Sam Harris argue in very similar terms today.
He admired the intellect not for its ability to submit to God but for its ability to understand the world and the universe and its irrepressible inquisitiveness.
He also doubted that Muhammad was a prophet.