Umar ibn Al-Khattab was known as Al-Farooq (sometimes spelled, Al-Faruq, Al-Farouque) which means the Criterion; the one who distinguishes between right (truth) and wrong (falsehood). He was given this name by the Messenger of Allah (ﷺ) due to Umar openly showing his Islam in Makkah and through him Allah distinguished (farraqa) between disbelief and faith.
Ibn Al-‘Abbas, related that he had asked ‘Umar ibn Al-Khattab why he had been given the epithet of Al-Farooq, ‘Umar replied: “After I had embraced Islam, I asked the Prophet (ﷺ): ‘Aren’t we on the right path here and Hereafter?’ “
The Prophet (ﷺ) answered, “Of course you are! I swear by Allah in Whose Hand my soul is, that you are right in this world and in the hereafter.’ “
I, then asked the Prophet (ﷺ), “Why we then had to conduct clandestine activism.” He (ﷺ) replied, “I swear by Allah Who has sent you with the Truth, that we will leave our concealment and proclaim our noble cause publicly.”
We then went out in two groups, Hamza ibn ‘Abdul-Muttalib leading one and I the other. We headed for the Haram in broad daylight when the polytheists of Quraysh saw us, their faces went pale and got incredibly depressed and resentful. On that very occasion, the Prophet (ﷺ) attached to me the epithet of Al-Farooq.