God, oddly enough, has paid a deposit down on us. In fact, he has invested a lot in us. Through the Old Testament, God kept his word and stood by his people though they were rarely faithful. He also invested the lives of the Old Testament prophets, the life of John the Baptist, and the life of his son Jesus. We often say, "Jesus paid the price," or "The price has been paid in full." The life, death and resurrection of Jesus was the last installment payment to be made.
But then, after all that has been done, St. Paul says, God has given us the Spirit as a deposit, as a down payment of what is to come (2 Cor 5:5). If we have already been bought with the blood of Christ, if the final payment has been made, why does God make a new down payment? St, Paul must not mean a deposit in the sense of buying something, he must mean a deposit in the sense of a sample. Your local bakery will give you a sample taste of a new muffin in hopes that you will buy the whole thing. At a department store you may try a new fragrance. These samples are enticements, lures, or bait to get you to buy something.
The Holy Spirit that we have experienced thus far in our lives is therefore only a small part of the whole experience of the Spirit that is to come. The deposit of Holy Spirit in our lives guarantees that there is more to come. With God there is always more. Come, Holy Spirit.