"The Spirit is God's guarantee that he will give us the inheritance he promised and that he has purchased us to be his own people. He did this so we would praise and glorify him."
I really like things in life that are for sure. Unfortunately, most of life is uncertain. There aren't too many guarantees in life. Perhaps that is why I like this verse. Because, with God, there are certainties, there are guarantees. The Holy Spirit that lives in you once you have accepted Christ is God's guarantee that we really do belong to him and that the inheritance that he has promised us (eternity with him) really will happen.
I'm glad he did this. You see, when we accept Christ, we receive the Holy Spirit to live in us. God has given us the Holy Spirit to be our helper, to be Christ within us. Every Christian has the Holy Spirit in them. This means that every Christian has the guarantee from God that they have been purchased and that they will receive their inheritance. So when we face doubts about our salvation or whether or not God actually heard us when we called on him, we can rest on this: that God has guaranteed, by sending the Holy Spirit, your salvation and your inheritance.
Why did he do this? The answer is rather simple. He did this so that he would be glorified and praised by us. This is why he gives us a guarantee. So that we will praise him. He loves to hear the praises of his children. So today, praise and bring glory to God for what he has done by saving you and for his promise that it will happen. No if, ands, or buts. It will happen.