After Jesus' Resurrection and after many days of appearing to Believers to prove it, before Jesus ascended to be with the Father (until His promised return), He promised He would not leave us alone, but would send us a "Helper" - God's Holy Spirit:
"15 “If you love me, you will keep my commandments. 16 And I will ask the Father, and he will give you another Helper,[f] to be with you forever, 17 even the Spirit of truth, whom the world cannot receive, because it neither sees him nor knows him. You know him, for he dwells with you and will be[g] in you." (John 14)
A lot of Christians know little about God's Holy Spirit, even when having considerable knowledge about the Father and the Son (Jesus). And this is because many churches fail to teach adequately about Him. God is ONE - He is ONE God of THREE Persons - EACH fully and equally God in ever way, but their roles are different. For us, that's hard to understand - a Trinity that is yet ONE God - and yet that is what the Bible prolifically teaches! Christianity is NOT polytheistic - Christians are monotheists!
Here, Josh McDowell's son, Sean, and a theologian discuss WHO the Holy Spirit is, what He does, and how He works - as well as take a look at some common misconceptions about Him:
https://www.youtube.com/watch?v=BXSIAGFkRTY