The relationship between Baha’u’llah and the heart is a tricky one to grasp because it involves spiritual concepts of nearness and distance and not everyday ones. Typically, a person on the other side of the world is considered far away, and the person next door is considered near. But when looked at from the point of view of spiritual nearness and distance, these relationships can be reversed. That is, it might be that a person on the other side of the world is spiritually close, whereas the person living next door is spiritually distant.
One way to look at the connection between Baha’u’llah and the heart is to liken it to the connection between a mother and her unborn child. The mother’s body encompasses the entire being of the child. The two are not separate in any way. And yet, despite this, the child is a separate person. From a physical perspective, the mother and child are intimately close, but from the perspective of conscious awareness, they are distant in that the child has no concept of the mother. This distance is overcome only when the child is born and develops an awareness of the mother.
Baha’u’llah uses two analogies drawn from nature to illustrate this state of affairs. In the first, he likens the person to a leaf on a tree, which is unaware of the tree and the fact that it sustains the leaf’s existence. In the other, he likens the person to a fish in the ocean, which is unaware of the water it swims in and the fact that the waters sustain its life. (Baha’u’llah: Summons of the Lord of Hosts) These analogies illustrate how the human being is already merged with the reality of Baha’u’llah but is nevertheless distant from him due to lack of awareness.
All souls that turn their faces toward the shrine of certainty are reckoned among the people of nearness, while all who turn away are remote. Proximity to God is devoid of remoteness, if the people do not remain distant. The root of the tree of nearness is planted in the earth of this utterance that descended from the heavens of the All-Merciful, and its branch encircled the worlds. When the sun of the Name, ‘the Near,’ dawned forth from the Orient of reunion, it encompassed all the horizons. Spiritual closeness is prior to and more near than physical proximity. For it can be seen that often persons who were with God morning and night appeared to be thus honored, but God refused to recognize their nearness, because they lacked spiritual closeness. God is Knowing and is the Concealer, and if a soul dwells in the remotest lands, yet gazes toward God, then he is accounted among the near ones. Blessed and excellent is the soul that has both bestowed upon it.Baha’u’llah, Commentary on a Verse of Sa’di, paragraph 4
(This is a passage I wrote for my book in 2020, but decided not to include. )