According to a certain sociology model, in each person there exists an I, a Self and a Me. As I understand it, the I is the person, as they truly are, looking out on the world; the Self is their reflective self-image, their idea of who they are; and the Me is the person they present to the rest of the world. The Self originates from the I, and the Me comes from the Self and the I together. To help explain, I will use an example:
Say someone decides to take a selfie. The I takes the picture; the Self is captured/expressed within the picture, reflecting the I; and the Me posts the picture online, sharing the life of the person with others.
Now to talk Trinity. God knows Himself perfectly, and cannot be deceived, so God’s Self is perfectly identical with God’s I, and so both are wholly God. God’s Me, by which God presents God to the world, is the fulness of God, because God is all good, and loves Himself perfectly, and so has no bad to hide and no good to forge. God’s Me, then, is the love of the I for the Self and the Self for the I, and proceeds from both the I and the Self. The Father corresponds to the I, the Son corresponds to the Self, and the Me corresponds to the Holy Spirit. They are perfectly united, and yet truly distinct. Three in One and One in Three.
The I, Self and Me model can also, I believe, help to explain how people come to be united, and so, by extension, how we come to be united with God. As the Me shares the person’s inner life, if it is accepted, those it is shared with begin to take part in this inner life; they spend more and more time with the person, talking more and more intimately. As this goes on, the outsider grows to be ever more closely identified with the person, entering into the person’s idea of their Self; they consider each other as themselves (even to neglecting their own self), and take ever more joint selfies. And as they enter into the person’s Self, they are even drawn into the I, where they live and work as one, with a single set of desires.
Between people, this process can be bumpy to say the least. Sin and the stain of sin place barriers between us at each point, and our own I, Self and Me are not perfectly united. But with God, He removes all such barriers along the way, and brings us to internal unity. And as a wonderful bonus, this removes the same barriers from our relationships with others, enabling true communion on earth also.
We see this growing into unity with God throughout the scriptures, and especially in the Blessed Virgin, the Spouse of the Holy Spirit, whose flesh was made the flesh of the Word, who was united most profoundly to God and so made Queen of Heaven. The Holy Spirit is credited with making the Church the Body of Christ, and with consecrating the Holy Eucharist. It is by the Holy Spirit that we know God’s life, so that we are united to Jesus (especially on the cross), so that we are offered in perfect obedience to the Father.
May God the Father, the Son, and the Holy Spirit bless you