It is not that it isn’t understandable. It is that there are several ways to interpret that “we are One”.…Jesus would have never gone on and on with One - were that to be non-understandable…
Sameness and oneness are not the same thing, and it is precisely part of the problem.JESUS? Often speaks of His Father in a manner reflecting their Sameness (Oneness)
Arianism took the “we are one” to mean that the Son was like the Father (of a similar essence – sameness) but not consubstantial with the Father (of the same essence – oneness). This position is not Trinitarian.
Adoptionism believed that the Son was a human being made one with the Father through the descent of the Holy Spirit – this position is not Trinitarian.
Modalism understood that oneness to mean that the Father, the Son and the Holy Spirit are not really persons, but just different modes of being of the same deity – that is, that the persons do not really exist in themselves, they are just different ways God reveals himself.
The list could go on, but none of these positions, while they all try to explain what it means when Jesus says that He is one with the Father, are the orthodox understanding of the Trinity. None of them accurately describes the way Father, Son and Holy Spirit really are One. Trinitarian theology is a minefield where it is extremely easy to stray away from the orthodox comprehension, that is from the most faithful description of God’s being our limited understanding is able to produce.
That orthodox belief in the Trinity developed over the first centuries of Christianity and was finally formalized at the first council of Constantinople in 381.
When one has said that Father, Son and Holy Spirit are one, one hasn’t really said anything yet. One has just begged a huge question, and the answer to that question does not lie in the Bible, although this is where its roots are.