Sentence:  if an evil spirit counterfeits the presence of God, and comes "upon" the man as an influence only, it may be described as <WSD>obsession</WSD>; but when a footing is gained "in" him, it is "possession," because the obsessing spirits have gained access, and possess the ground they hold, up to the extent of the ground given. 
Meaning of the word marked with <WSD> tags:  Influence or control by evil spirits without possession.