In the name of Allah, the Beneficent, the Merciful

حـم - تَنزِيلُ الْكِتَـبِ مِنَ اللَّهِ الْعَزِيزِ الْعَلِيمِ

(Ha-Mim. By the manifest Book.) means, plain and clear in both wording and meaning, because it was revealed in the language of the Arabs, which is the most eloquent language for communication among people. Allah says:

A Preacher must not give up his preaching due to disappointment

أَفَنَضْرِ‌بُ عَنكُمُ الذِّكْرَ‌ صَفْحًا أَن كُنتُمْ قَوْمًا مُّسْرِ‌فِينَ (Shall we remove the advice away from you because you are a transgressing people? - 43:5) The meaning is that We will not give up advising you through the Qur'an, no matter how rebellious and disobedient you may become. This tells us that the person who is engaged in preaching and inviting others towards Islam should carry the message to everybody; he must not give up preaching to some people or any group simply because they are non-believers, highly irreligious, sinners and transgressors.