﴿ فَلَمَّا جَآءَهُم مُّوسَىٰ بِـَٔايَٰتِنَا بَيِّنَٰتٍۢ قَالُوا۟ مَا هَٰذَآ إِلَّا سِحْرٌۭ مُّفْتَرًۭى وَمَا سَمِعْنَا بِهَٰذَا فِىٓ ءَابَآئِنَا ٱلْأَوَّلِينَ ﴾

“But as soon as Moses came unto them with Our clear messages [Pharaoh and his great ones] said: “All this is nothing but spellbinding eloquence devised [by man]: and never did we hear [the like of] this, [nor has it ever been heard of] in the time of our forebears of old!””

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.