When they started teaching that Jesus wasn't the mediator for all true believers but only for their little class, they were laying the groundwork for being the conduit for holy spirit.
I don't understand's people objection to the teaching of Jesus being the mediator for the anointed only. Moses was mediator for the Israelites only, so Jesus can be the mediator for the spiritual Israelites only. People don't need Jesus as a mediator to get life, or forgiveness or prayers passed to his Father. Objecting to this teaching is not understanding what the priestly arrangement is. Jesus as high priest gives the atonement for all. The atonement is what forgives sins and cleanses. The priest also offers up prayers for all like incense. Mediator refers to covenants which means both sides have a give and take. Why would you want to be part of that?