God is not, like creatures, made up of parts. God is spirit, without bodily dimensions. Firstly, no body can cause change without itself being changed. Secondly, things with dimensions are potential of division. But the starting-point for all existence must be wholly real and not potential in any way: though things that get realized begin as potential, preceding them is the source of their realization which must already be real. Thirdly, living bodies are superior to other bodies; and what makes a body living is not the dimensions which make it a body (for then everything with dimensions would be living), but something more excellent like a soul. The most excellent existent of all then cannot be a body. So when the scriptures ascribe dimensions to God they are using spatial extension to symbolize the extent of God’s power; just as they ascribe bodily organs to God as metaphors for their functions, and postures like sitting or standing to symbolize authority or strength.
Thomas Aquinas, Summa Theologiae: A Concise Translation