Because Christ is a symbol of the self, in other words, the projection of the collective self (of believers).(1)
Thus, by studying biblical texts, Jung can reveal the phenomenology of the self.
Was Jung religious person?
Yes, he says in an interview. He doesn't need to believe like a common
devotee because he knows. God is the archetype!
Of course, Jung's psychologizing attitude to religion experience attracted attacks upon attacks.
But his reply was not unexpected: as a psychoanalyst, he is
interested in the experience of God in our psyche (soul).
Jung was the son of a Protestant pastor.
In this regard, he tells us how astonished he was when, asking
his father if he had seen Christ, the latter was totally confused because he had not imagined that Christ could be seen.
This incident, which outraged the young Jung, followed him all
his life, provoking him to prove how God can be known and experienced even, if not directly, through the archetypes.
If he had known the works of Gregory Palamas, he would have learned about the philosophy of
uncreated energies and would have been surprised to find that the psychologization of religious experience is undoubtedly a theological subject.
Because, yes, God can be known through archetypal
phenomenology and experienced, therefore, as an archetype.
These ideas cannot be supported in the Western world dominated by Catholicism and Protestantism. Unfortunately,
Jung did not know Orthodox Christianity, which would have offered other perspectives to his studies.
Notes:
1. And thus he approaches oriental mysticism which claims
that God is inside, in the (mystical) heart of each one, according to Ramana Maharhsi.