The Apostles taught that we are sealed in the HS at baptism, that we become the temples of God, because His Spirit dwells in us. We expereince this indwelling through our conscience, directing our thoughts.
I agree that neither evil or good spiritual influences are a “by product of the brain”, but both evil and good work through our human nature (mind and heart) to “speak” to us.
Rom 2:15-16
5 They show that what the law requires is written on their hearts, to which their own conscience also bears witness; and their conflicting thoughts will accuse or perhaps excuse them 16 on the day when, according to my gospel, God, through Jesus Christ, will judge the secret thoughts of all.
