Commentary
Using foul or harsh language is not correct, even against infidels
The first verse (53) prohibits Muslims from talking harshly to disbelievers. It means that a hard line should not be taken unnecessarily, and should there be the need, even killing would be in order - as succinctly put in two lines of Persian poetry:
کہ بےحکم شرع آب خوردن خطاست وگر خون بفتوٰی بریزی رواست
Whereas, without an injunction of the Shari` ah, to drink water is a sin,
In another case, to shed blood under Fatwa is permissible!
Since it is possible to suppress the aggressive might of disbelievers and their hostility to Islam through fighting and killing (as an inevitable measure of defense), therefore, it is allowed. However, by using foul language and harsh words, neither a castle stands conquered nor anyone guided right, therefore, it has been prohibited. Imam al-Qurtubi said that this verse was revealed in the background of an event relating to Sayyidna ` Umar ؓ . It so happened that someone used foul language against Sayyidna ` Umar ؓ who retaliated harshly and wanted to kill him. This posed the danger of a war breaking out between their two tribes. Thereupon, this verse was revealed.
And according to the interpretation of Imam al-Qurtubi, this verse contains a guideline for Muslims as to how they should address each other. It suggests that they should not speak harshly at the time of mutual difference, for this is a trap door through which the Shaitan pushes them into mutual infighting and disorder.