In Him you also trusted, after you heard the word of truth, the gospel of your salvation; in whom also, having believed, you were sealed with the Holy Spirit of promise, who is the guarantee of our inheritance until the redemption of the purchased possession, to the praise of His glory. Ephesians 1:13, 14.
God is true to his word. He keeps his promises. But, that was not enough for him. He gave us a down payment, a deposit, so we could be sure of our inheritance and look forward to the full realization of our salvation. He gave us the Holy Spirit. When God buys us out of slavery to our mortal bodies and redeems us into our eternal home with bodies that will not grow old or decay, we won’t need the deposit; we will have the full inheritance. But until then, until we go to him, isn’t it amazing that he makes his home within us!