What Maulana Rumi (Rahimullah) meant is that the knowledge of Allah ﷻ, root of divine knowledge lies in our Soul (Nafs) and the purpose of each individual’s existence is to know their soul (nafs) so that divine blessings descends upon it and they gain that knowledge which assists them to come closer to Allah ﷻ.
Unfortunately most of us keep ourselves or our souls (nafs) busy with material world with hardly any or very less concentration on matters of hereafter which leads us to keep our souls blocked and covered from receiving the divine and that is where we are not only wasting our potential but also defeating the purpose for which we were created. Like the great Sufi Imam Ghazali said "air does not enter a pot full of water, so knowledge of Allah ﷻ does not enter a soul or mind if it remains filled up with things other than Him."
This saying of Maulana Rumi can be best understood in the light of the following Quran verses: (Al Quran 91:7–10) by the soul (nafs) and how He formed it and inspired it [to know] its own rebellion and piety! The one who purifies his soul succeeds and the one who corrupts it fails.
Please also read something I wrote on Soul: The Mystery of Human Soul