Who are these "most people", may I ask? FYI, the sentence is grammatically correct and both "that" and "which" are not conjunctions.IMO, most people would write this sentence as .... as that fills the universe or as which fills the universe. Why the authur used both conjunctions?
"It is my belief that mind is made up of the same universal energy as that which fills the universe." = "It is my belief that mind is made up of the same universal energy as (the unversal energy) which(rel.pron) fills the universe."