There are many correct answers to this question: the power of the Word of God, the filling and movement of the Holy Spirit, the giftedness of the preacher, the eagerness of the people to hear, all could be mentioned when this question is asked. 

Yet, I want to mention one answer that is commonly overlooked when considering the power of preaching and what produces a moving sermon that brings spiritual fruit from God.  

I think this one overlooked aspect of powerful preaching is best summarized by the 19th-century English pastor Archibald Brown:

Oh, brethren and sisters, I would to God I could speak to you this morning as I would. 

I only wish I could make this text blaze away before your eyes as it has before my own. I would that its tremendous force might be realized by you, as it has been felt in my own heart before coming here. 

Oh, how it would shake some of you out of your selfishness, out of your worldliness, out of your pandering to the maxims of this world.

Brown’s words capture well an essential element to a powerful sermon—that is, the preacher first be deeply affected by the word he steps into the pulpit to preach. 

Before the preacher can persuade any sinner to turn to Christ, he must first be persuaded himself. 

Before the preacher can convince any Christian to trust in the promises of God, he must first believe those promises.

Pastors, as you prepare to preach God’s word and feed the souls of your people this week, make sure that word you study has changed you. 

Make sure it is a part of you and that you truly believe what you are preparing to preach so that you are able to preach with an earnestness that only comes from someone who has met with God and experienced his help.