Encouraging good and discouraging bad is a vital requirement of being a Muslim as mentioned in Surah-Asar. Usually if a Muslim tries to perform this “MUST duty” with wisdom, few people might misunderstand him and think that he is trying to show his dominance & proving others wrong only for boosting his own ego, while only Allah (swt) knows his true intentions, which even the preacher himself doesn’t know100% until death.
Even if the biggest scholar says that his heart is pure of evil (show off, pride,lust..etc.) and his Imaan is alright, he himself is the biggest Shaitaan (devil) because only Prophets have the right of such an incredible claim.
Now if a less practicing Muslim says to a preacher, “Go and preach to non-Muslims” He is forgetting that preaching is actually a reminder and correction of oneself. Because even a believer needs reminders, for example:
O ye that believe! fear Allah and believe in His apostle…(57:28-29 Surah Al-Hadid).
Of course if one is a believer, it is already known that he fears punishment of Allah (swt) and believes in His apostle, but here it is emphasized because our “fear of Allah (swt)” is weaker than fear of worldly problems and our “believe in Allah’s apostle” is not up to the mark that is why we follow very less of “his ways”
Just like if we say to a child (wearing dirty clothes)," Go, and wear clothes". Of course he is already wearing clothes, but it is evident that we meant to tell him to change it with clean ones…" The child, who has got used to dirty clothes and no longer think of it to be dirty, replies, " My clothes were perfectly clean when I bought them" but he forgets that without taking much care they would get dirty with time, which requires cleaning. If he has got used to dirty clothes, doesn’t mean that his clothes can be considered as clean as they were when he bought them…