Why is the Holy Spirit's presence in a believer called God's down payment?
The Holy Spirit's presence is called God's down payment because it guarantees that He will complete the whole transaction of redemption. The soul and spirit of a believer are already redeemed, but the body remains unredeemed under the curse. The Spirit assures that body, soul, and spirit will be redeemed together for eternity.
Les teaches that no human being can function as a complete entity unless he is body, soul, and spirit. For the believer, the soul and spirit have already been redeemed, but the body is still unredeemed, still in the flesh, and still under the curse. The very presence of the Holy Spirit in our life as a believer is God's down payment that He is going to complete the whole transaction. He will not redeem only the soul and spirit; He will redeem body, soul, and spirit together. All three must be brought back together before we can spend eternity in the presence of Christ. Les then ties this to Romans 8, where the sufferings of this present time are not worthy to be compared with the glory that shall be revealed in us, and creation waits with earnest expectation for the manifestation of the children of God.