This is basic for understanding the power of the Bible.  Here is a basic explanation of how this works, taken from a post on a seminary’s website:

But how did the divine Word come to us from the inner being of God, as he breathed it out? If 2 Timothy 3:16 speaks to the product, then 2 Peter 1:21 speaks to the process.

“For no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.” A more exact rendering of the Greek form of the second clause would be, “… but by the Holy Spirit being carried/led, men spoke from God.” In other words, the Holy Spirit was the divine Agent who carried or led or conducted the authors of Scripture so that what they were writing was God-breathed Scripture.

The Greek participle translated “carried” (φέρω, pherō) simply means to bring something like an object or a message (hence, “carry”) or lead someone from one place to another. But because the word “spirit” can also mean “wind” or “breath,” authors have sometimes suggested that Peter is thinking that the authors are like ships driven by the Spirit who filled their sails.

John MacArthur is relying on this tradition when he explains,

As those godly men were carried along by the Holy Spirit, He superintended their words and used them to produce the Scriptures. As a sailing ship is carried along by the wind to reach its final destination, so the human authors of Scripture were moved by the Spirit of God to communicate exactly what He desired. In that process, the Spirit filled their minds, souls, and hearts with divine truth— mingling it sovereignly and supernaturally with their unique styles, vocabularies, and experiences, and guiding them to produce a perfect, inerrant result (Strange Fire, 2013, p. 223).

What This Means for Your Preaching

Several things come to mind:

• First of all, it means that preaching can have power.  And this is not because you are so good at it, but because God is at work to fulfill his plan for the people who hear you.
• Secondly, it means that when you feel inspired in your preparation, know that the Spirit is at work.  He is speaking to you so that he can speak through you.
• Thirdly, remember that the Holy Spirit Inspires the Word for your preaching.

An Example

I realized this when I was in seminary.  I accepted an invitation to preach in a church that was right across the street from the high school that I attended.  In other words, there were going to be a lot of people in the audience that I knew.  However, they knew me when I was planning to study music in college.  Now I was in seminary.  And I had the unmitigated gall (as some of them might think) to stand up and speak for God.

So, I prepared, praying for inspiration so that I could be inspirational.  Some of my motives weren’t very good.  I wanted to impress these people with my preaching–and ultimately, to impress them with my ability to preach.

The sermon that day was on a part of what I believe to be the greatest chapter in the Bible: Romans 8.  During my preparation, I found myself excited by the truths that I was reading and writing about.  That excitement I was able to bring the the preaching that day, and thanked God for the reality of God’s revelation.

Conclusion

When you are preparing to preach, remember that it is the Holy Spirit inspires the Word for preaching.  He did this so that you can touch the heart and mind of God in your preparation.