Zac Poonen emphasizes three essential conditions for being a spokesman for God as illustrated in Jeremiah 15:16-21. First, one must find joy and delight in God's word, prioritizing it above all else. Second, it is crucial to avoid the company of those who indulge in frivolity, as discipline in solitude with God is necessary for spiritual growth. Lastly, Poonen highlights the importance of speaking words of faith rather than expressing doubt or complaint, as God desires His messengers to communicate truth and goodness. By adhering to these principles, believers can become effective mouthpieces for God.