Why is the truth of Christ in us believable to believers even though the world considers it nonsense?
The indwelling Christ is believable to believers because it is practical and experiential; they know it has happened. They love and know the Word, so Christ in them—by the Holy Spirit, the third person of the Trinity—is as plain as day, though the world understands none of it.
Les explains that just as a husband and wife come together and become one, Christ and this body of believers become one—totally integrated. Not only are we in Christ, but He is in us. The world knows nothing of this; someone flipping through channels might think the teacher is a kook and wonder what he is talking about. But for those who love and know the Word, it is as plain as day. It is so believable because it is practical and experiential; they know it has happened. Les points to 1 Corinthians 3:16, where Scripture expects believers to know they are the temple or dwelling place of God and that the Spirit of God dwells in them. Since Christ is bodily at the Father's right hand and cannot leave that bodily, Christ in us is by the third person of the Trinity, the Holy Spirit. Les also notes that when Scripture repeats something in short order, it is emphasizing the point.