William MacDonald emphasizes the seriousness of defiling the temple of God, which refers to the local assembly of believers. He explains that Paul warns against those who destroy the unity and holiness of the church, as such actions lead to God's judgment. The sermon highlights the dangers of gossip, factionalism, and personal conflicts within the church, which can lead to its downfall. MacDonald reminds the congregation that both individual believers and the collective assembly are sacred, and tampering with them is a grave offense. Ultimately, he calls for unity and purity within the church to honor its divine purpose.