Now - this "mark" of the Holy Spirit - this "down payment" language speaks of "earnest money" - A guarantee for full payment to come later. The Greek word for earnest is "arrabon" - The modern Greek word for this is "arrabona" and it is the word for ENGAGEMENT RING. Do you see that, beloved? This language is the language of BETROTHAL. There's is in this a PROMISE that "I will come back for you!"
In the Jewish Wedding customs, when someone is betrothed, they are considered married all except for the consummation of the marriage. This is why Joseph considered divorcing Mary when he found out she was pregnant. They were not yet married - the marriage had not been consummated - but they were bound to one another nonetheless. Just like that - we are BETROTHED to Christ now and we have this promise (by the guarantee, "bridal dowry" so to speak of the Holy Spirit) that guarantees that what He has purchased (US!) will be bought in FULL at the time of His return - at the consummation of all things! This being "IN HIM" and Him being "IN US" will be made complete when He comes. We are marked with a seal as His own. The Holy Spirit is a guarantee that the things that are realities now in a Spiritual sense (being with with Christ in the Heavenly Realms NOW, etc.) will become more real - it will be made complete when He returns for us, His bride! The wedding, the consummation, the completion WILL COME! All things will be fulfilled!