﴿ وَقَالَ مُوسَىٰ رَبِّىٓ أَعْلَمُ بِمَن جَآءَ بِٱلْهُدَىٰ مِنْ عِندِهِۦ وَمَن تَكُونُ لَهُۥ عَٰقِبَةُ ٱلدَّارِ ۖ إِنَّهُۥ لَا يُفْلِحُ ٱلظَّٰلِمُونَ ﴾

“And Moses replied: “My Sustainer knows best as to who comes with guidance from Him, and to whom the future belongs! Verily, never will evil­doers attain to a happy state!””

A sermon should have high degree of eloquence هُوَ أَفْصَحُ مِنِّي لِسَانًا (He is more fluent in his tongue - 28:34). This verse points out that a high degree of oratory and eloquence is desirable for sermons and preaching, and there is no harm if one takes training in that.