John 14:15-31
And I will ask the Father, and he will give you another Helper, to be with you forever, (John 14:16 ESV)
Jesus' time with his disciples was drawing to an end. Soon he would no longer be with them in the common way we think of when we talk about companionship. Instead, Jesus said that the Helper was going to come. That actually by him leaving it would empower the disciples to serve God in even more incredible ways. The Helper Jesus was speaking of is the Holy Spirit.
Traditional (Orthodox) Christianity has always held the belief of a triune God, that means God expressed through three persons. It is all rather complicated sounding, but it is actually really simple. There is God the Father, God the Son, and God the Holy Spirit.
There are a ton of varied beliefs that revolve around the Holy Spirit. Some movements have very exuberant ideas about what is involved as we interact with the Holy Spirit, while some try not to acknowledge the Spirit at all. There are a few things that are quite clear about the Spirit.
All throughout scripture we see God performing powerful deeds. The Holy Spirit is always at work in these stories. Jesus performed miracles by being filled with the Spirit at his baptism. Many Old Testament heroes did mighty and amazing things under the power of the Spirit of God.
Salvation is a work of the Holy Spirit. After salvation the Holy Spirit continues to urge us toward personal growth in Christ. The Holy Spirit assists us in the ability to better fight temptation. Awareness of the Spirit makes us more sensitive to the way God sometimes chooses to communicate with His kids. The Holy Spirit empowers us to demonstrate fruitful Christian lives. The Holy Spirit sometimes works through us to do the miraculous.
Those are all incredible things. They make life with God outstanding. Indeed, I can not imagine trying to live this God-life without the instruction and influence of the Holy Spirit.