1 Corinthians 14:9
So likewise ye, except ye utter by the tongue words easy to be understood, how shall it be known what is spoken? for ye shall speak into the air.
Note 14 at 1Co 14:9: Paul had drawn on physical examples to make his point (see note 13 at 1Co 14:6). He continued his comparison by pointing to musical instruments. They play melodies, not just disconnected notes that don't harmonize (1Co 14:7). In war, the trumpeter (or bugler) sounds precise notes to signal instructions to the troops (1Co 14:8). Likewise, in church, we need to limit our utterances to those that everyone can understand, so they can be edified.
Note 15 at 1Co 14:9: Paul was speaking about refraining from speaking in tongues in church, unless there is an interpreter. This is so that everything done in church will edify the whole group, not just an individual. However, this same principle can be applied to other activities in the church besides speaking in tongues.
Someone who preaches at an intellectual level that is above everyone else in the congregation is certainly not uttering "words easy to be understood." False intellectuals have bored many a congregation to sleep. Truly smart communicators are so easy to understand that everyone can grasp their meaning. This is also the logic behind special ministries to different age groups. They must understand the Word, or Satan will come immediately and steal the Word of God out of their hearts (Mt 13:19).