Question: Sectarian Muslims quote Hazrat Umar's saying, "We used to judge other by their outer behavior and used to leave what was hidden". Based on this they say that we too will declare Muslims as kafir, mushrik, munafiq, jahannami etc based on what is apparent in a person".
How should we respond?
Answer: What Hazrat Umar said was in the matter of judgement in worldly affairs. Similarly, even today, a qazi will judge based on external identity. If someone says that he is a Muslim then ruling, judgement, administration will be done based on that. For example, in your passport if you have mentioned your religion as Islam then you will be treated as Muslim for all administrative purposes. So rulings on worldly affairs will be based on what you claim. This is the context of saying of Hazrat Umar.
However, in the sight and records of Allah who is a Muslim, who deserves paradise, who deserves hell, who is munafiq - it is known only to Allah. And that is why there is a judgement day. If human beings start becoming 'malik e yaum i ddeen' then what is the need of God to do absolute judgement in akhirah?
This attitude of Muslims that we will give fatwa of kufr and shirk has lead to the cancerous problem of takfeer wherein every Muslim group declares other group's Muslims as deviated and jahannami. Quran is free from such erroneous approach.
"By Allah, though I am the Apostle of Allah, yet I do not know what Allah will do to me". Prophet Muhammad. Sahih Bukhari, Volume 5, Book 58, Number 266
Following two events from the life of Hazrat Umar - further attests to our point:
A. One day while he was holding the office, caliph, Umar knocked at the door of Huzaifa, a
confidant of the Prophet. In a weary and gloomy mood he said to Huzaifa, “Swear in the name of Allah that you will speak the truth”. Huzaifa implored, “what are you going to ask, O Emir of Muslims”. Umar said, “No, swear first that you will give me a truthful account.” Then in a state of extreme commotion he said, “The Holy Prophet has told you the names of all the hypocrites of Madina. Speak honestly if my name figures in the list.”
This event clearly differentiates between "how people understand you" and "what is your exact status in the sight of God".
B. Hazrat Umar used to say: "Belief is the name of the intermediary state between hope and fear. If I come to know that every person except one will go to paradise I will have the fear of being that person; and if (I come to know that) every person except one will go to Hell, I will have the hope in Allah’s benediction of being that one lone person”.
Above words of Hazrat Umar - clarifies everything. The ultimate judgement is with Allah regarding who is momin, who is kafir and who is munafiq. We can just quote those qualities that are mentioned in Quran.