In Jeremiah 15:16-21, we see three conditions to be a spokesman for God.
First of all: "Lord, I found Your words, and Your word has become for me the joy and delight of my heart" (Jer. 15:16). God's word must be the joy and delight of your heart. Just like a businessman has joy in making money, your joy must be in getting into God's word. Many want to be preachers today, who do not spend time digging into God's word, and who don't look at God's word as the joy and delight of their hearts.
Secondly: "I did not sit in the circle of merrymakers" (Jer. 15:17). When other people in Judah were partying and having fun, Jeremiah went away by himself to be alone with God. If you don't discipline yourself to stay away from the jokers of this world, you can never be a spokesman for God. I am not saying that humour and clean jokes are wrong. But many Christians don't know where to stop in such matters - they are perpetual jokers. Jeremiah made sure that he didn't spend time with such people.
Thirdly: Jeremiah made a complaint to God in verse 18 saying, "Lord, why have You let me down? You have been to me like a deceptive stream, like unreliable water. I come to the stream thinking there is water there and there is none. You have let me down." The Lord said, "Don't ever say things like that to Me."the Lord rebuked Jeremiah for speaking words of unbelief (Jer. 15:19). God never lets us down. He is not like an unreliable stream. Jeremiah was depending on his feelings and looking at his circumstances. The Lord told him, "If you return to Me, and get rid of the habit of speaking worthless words (like the ones you just spoke - idle, useless words and words of unbelief) and ensure that you speak only precious words (of faith and goodness), then you will be My mouthpiece."
How many of you want to be God's mouthpiece? I am not talking about being a preacher of dead sermons that come from some book you read, but being God's spokesman. If you want to be that, then don't waste time in useless company, but redeem your time and spend it digging into God's Word. Let that be your delight. Get rid of useless conversation and speak words of faith always and only good words in all your conversation. Then the Lord will make you His mouth. There is no partiality with God.