1 John 4:6 

We (who teach God’s Word) are from God (energized by the Holy Spirit), and whoever knows God (through personal experience) listens to us (and has a deeper understanding of Him). Whoever is not of God does not listen to us. By this we know the spirit of truth (motivated by God) and the spirit of error (motivated by Satan).

John recognizes that sound preaching comes from a place of guidance by the Holy Spirit. And those who know God through salvation recognize sound preaching and are not swayed by false teachings and opinions against the knowledge and Word of God. We are to keep our minds sound through the Word and let wisdom found in the Bible keep us clear of error.