When Christ completed His earthly ministry and ascended back to heaven, the Father sent the Holy Spirit to minister in His place. At the time of salation, the Holy Spirit comes to live in us. Though we receive all of Him, we don't always allow Him to have all of us. Our busy lives are so full of other things, that we often confine the Spirit to a dark corner and forget that He is even there.
These lessons are written with the hope that we will see the importance of the Holy Spirit in our daily Christian walk. "That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit." Romans 8:4