And this is where the “fullness” comes in, literally. When He comes in, He brings His “fullness” – infinite love, joy, peace, longsuffering, kindness, goodness, faithfulness, and so forth. His presence is His fullness. Filled with His fullness means filled with God’s presence and thus all that He is. It’s a spiritual union:

For we are members of His body, of His flesh and of His bones.

"For this reason, a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh."