Theophany is the word we use to describe the visible manifestations of God. The primary theophany was more specifically the appearance of Christ (Christophany) but there are other biblical examples of these theophanies.
One such example is found in Genesis 18:17 and starts out with “the Lord said.” Now every the “Lord said” is not a theophany but in this passage the Lord is walking and talking directly with Abraham.
This is the place where Abraham intercedes on behalf of Lot with the Lord negotiating him down to a promise of not destroying Sodom if ten righteous people can be found in the city.
The Lord and Abraham meet and talk face to face. What an amazing thing—certain beyond my comprehension—but fully reconcilable with the entire body of truth when I realize that the disciples did the same thing. They ate, slept, walked and talked with God—Jesus the Son of God—for a portion of their adult lives.
We know that Abraham spoke with a pre-incarnate appearance of the Son of God because Jesus later told the disciples that no man has seen the Father (John 6.46) except himself—He is the only man that has seen the Father.
So what we know to be true is all theophanies are christophanies regardless of where in Scripture they occur. Even in the Garden of Eden God the Son was the one walking with Adam.
All this is only possible if God is a triune God. If he is One but NOT three persons then we can’t have Jesus saying no one has seen God and Abraham conversing with the Lord.
The doctrine of the trinity is essential to the reconciliation of truth with truth; without it the scripture contradicts itself and passages like Genesis 18:17 contradict 1 John 4.12.
This is precisely why the early church affirmed this critical doctrine in the Apostle’s creed and it became a standard of measuring whether someone was orthodox in their theology. Anyone who denies the trinity is a heretic.