As for me, I could just say I know it when I resonate with it. But I think we could do a better job explicating this depth thingy.
Best I turn to this passage in MotT as Tomberg has some interesting things to say on the matter:
“It should not be forgotten that Christian Hermeticism is not a religion apart, nor a church apart, nor even a science apart, which would compete with religion, with the Church, or with science. It is the connecting link (hyphen) between mysticism, gnosis and magic, expressed through symbolism —symbolism being the means of expression of the dimensions of depth and height (and therefore of enstasy and ecstasy), of all that is universal (which corresponds to the dimension of breadth), and of all that is traditional (corresponding to the dimension of length). Being Christian, Hermeticism accepts the cross of the universality, the tradition, the depth and the height of Christianity, in the sense of the apostle Paul when he said:
'That you, being rooted and grounded in love, may have power to comprehend with all the saints what is the breadth and length and height and depth, and to know the love of Christ which surpasses knowledge, that you may be filled with all the fullness of God. (Ephesians iii, 18-19)' ”So here Tomberg brings in the word enstasy which he equates with depth. It's not a word I'm familiar with, so let's see what Mr. Encyclopedia has to say:
“Enstasy (Gk., en-stasis, ‘standing into’). The experiences, or abolition of experience, arising as a consequence of those meditational, etc., techniques which withdraw the practitioner from the world, and even from awareness of the self. The word was coined in contrast to ecstasy. Examples are dhyāna, jhāna.”Fascinating concept, as it would appear to be a sober intoxication; with the emphasis on sober while intoxication equates more to the ecstasy (or height). Enstasy is not always proportional to depth of spiritual experience, but how fully you are living from your realizations, intuitions, and gnosis. This is good news for those of us who are not natural mystics!
If we go back to Tomberg, he also mentions breadth (universal inclusion) and length (reverence to tradition). It would seem to me a person with true depth would need to be somewhat immersed in these dimensions also.
So depth may be what grounds us as to how our whole being relates to existence itself. We also add in Paul's "rooted and grounded in love" and you get yourself a person who is awake, alive, integrated, curious, and (w)holy.
This person can't be measured, quantified, or mapped-out. They tend to be more analog than digital. And it's not someone who is easy to come by in today's world. But like can know like, so when cultivated within it can be seen without.
“To be in the depth is to be depth.” — A.H. Almaas