34 This same doctrine was set forth by our predecessor Leo the Great in these words: 'What principally contributed to the justification of mankind was that the only Begotten Son of God deigned to become the Son of Man, so that being God smoúsios to the Father, that is of the same substance, the same [person] should exist as true man consubstantial with his mother in the flesh; we rejoice over both these things, since only by both are we saved; we admit no division of the visible from the invisible, the corporeal from the incorporeal, the passible from the impassible, the palpable from the impalpable, the form of the servant from the form of God. For although he remains the one from eternity, he began to be the other in time; these two have met in unity and can have neither separation nor end' (St. Leo. Serm . 30, 6. PL . liv, 233S).
Source: Sempiternus Rex Christus (Vatican.va)