Holy Spirit of Christianity - Why I don't Believe in It
Firstly, God is one, since the ultimate efficient cause has to be one.
However, it isn't really problematic if God takes on different roles, or as Christians deem it, different prospons.
However, while the role of Jesus has been to make himself the bearer of the fruits of human sins, the purpose of the Holy Spirit is to convict one of sin and lead them to Jesus, and also to be the proof of faith in Jesus.
However, if one gets baptized and he continues to engage in sin as a result of his nature, then, if he doesn't correct it, the Holy Spirit will leave. And it is definitely true that people who are baptized do actually continue to engage in sins. This means one still has to maintain his actions even when he has received the Holy Spirit.
But if maintenance is still a necessity, what is the use of having a helper in the first place? Why not repent like Jews do? So by Occam's Razor, I do not feel the need for Holy Spirit to exist.