Pete McKenzie's sermon on Psalm 100 emphasizes the importance of joyful worship and gratitude towards God. He urges the congregation to serve the Lord with gladness, regardless of life's challenges, and to view their trials as opportunities to draw closer to Jesus. McKenzie highlights that joy is a choice and that true reverence for God involves recognizing His sovereignty and mercy. He stresses the significance of entering God's presence with thanksgiving and praise, and encourages believers to see their hardships as friends that lead them to Jesus.