Why does the Bible call the Holy Spirit a down payment?
The Holy Spirit is called a down payment because a down payment is only a holding pattern until something later consummates. God gives the Spirit as the earnest, or down payment, of the believer's future hope, when this body prone to die will never die. It holds the believer until that promised completion.
In the preceding verses, the hope of the believer is that this body prone to die is never going to die. Then verse 5 says that the one who wrought us for the selfsame thing has given unto us the earnest, or the down payment, of the Spirit. Les explains that the Holy Spirit inspired Paul to use the down payment picture because a down payment is only a holding pattern until something later consummates. The Spirit is therefore given as the present earnest of a future, final completion. Les also turns to Ephesians chapter 1, verses 13 and 14, where Paul writes that in Christ they trusted, placing their faith after hearing the word of truth.
Now he that hath wrought us for the selfsame thing is God, who also hath given unto us the earnest of the Spirit.