Umar ibn al-Khattab (RA) — 2nd Caliph
634–644 CE · The Rightly-Guided Caliphs
Under Umar (RA), the Islamic state expands into Syria, Iraq, Egypt, and Persia. He conquers Jerusalem on foot and refuses to pray inside the Church of the Holy Sepulchre so it could not be converted into a mosque — one of the earliest recorded acts of religious tolerance in conquest. He establishes the Hijri calendar, the diwan (welfare register), and is known as "Al-Faruq" (The Distinguisher between truth and falsehood).