Unless we understand what this really means – we will languish in doubt. When Scripture makes statements of fact concerning our Union with our Lord Jesus – we are called upon to believe it & then live in the light of it.
You can sense this in this Puritan prayer:
“You have taught me that Christ has all fullness and so all plenitude of the Spirit, that all fullness I lack in myself is in him, for his people, not for himself alone, he having perfect knowledge, grace, righteousness, to make me see, to make me righteous, to give me fullness; that it is my duty, out of a sense of my emptiness, to go to Christ, possess, enjoy his fullness as mine, as if I had it in myself, because it is for me in him; that when I do this I am full of the Spirit, as a fish that has got from the shore to the sea, and has all fullness of waters to move in, for when faith fills me, then I am full; that this is the way to be filled with the Spirit – first faith, then fullness, like Stephen – for this way makes me most empty and so most fit for the Spirit to fill.”