God the Holy Spirit
In the Bible account of how the church started (Acts Ch.2), we read that the Apostles were “filled with the Holy Spirit” (older versions use ‘Holy Ghost’) and thus empowered to begin preaching and healing in the name of Jesus. This power, enabled them to perform miracles and do amazing things like Jesus, and so was clearly God at work through them. To talk about God without including reference to this power was therefore clearly insufficient: yes, God was the unknowable creator, but He was also very much present and living in the world as a spiritual power. Thus, God was also the Holy Spirit in a way that was not the same as being the almighty creator, or as Jesus, the amazing person who had lived among them.