The Holy Spirit is the third person of the Godhead. Jesus refers to the Spirit as the Counselor. The Holy Spirit is what lives inside of us when we accept Jesus as savior. It is the living spirit of God Himself and His deposit in us that guarantees we are saved.
John 14:16 & 17 And I will ask the Father, and he will give you another Counselor to be with you forever– 17 the Spirit of truth. The world cannot accept him, because it neither sees him nor knows him. But you know him, for he lives with you and will be in you.
Matthew 28:19 & 20 Therefore go and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, and teaching them to obey everything I have commanded you. And surely I am with you always, to the very end of the age.”
Acts 2:38 & 39 Peter replied, “Repent and be baptized, every one of you, in the name of Jesus Christ for the forgiveness of your sins. And you will receive the gift of the Holy Spirit. 39 The promise is for you and your children and for all who are far off–for all whom the Lord our God will call.”