If thou couldst empty all thy self of self,
Like to a shell disinhabited,
Then might He find thee on the Ocean shelf,
And say — “This is not dead,” —
And fill thee with Himself instead.
But thou art all replete with very thou,
And hast such shrewd activity,
That, when He comes, He says — “This is enow
Unto itself — ‘Twere better let it be:
It is so small and full, there is no room for Me.”
“Indwelling,” Thomas Edward Brown
Kyriosity 
Madeleine L’Engle quotes this in A Ring of Endless Light … I’ve loved it for years 🙂
That’s where I first discovered it. Only she gets the wording and the poet wrong…as do lots of Inerwebz peeplz. I’m pretty sure this is the correct version, having researched it some years back.