How did the Son of God become visible in Old Testament times?
The Son, who is the image of the invisible God, stepped out from the invisible triune God and became visible in the Old Testament economy temporarily. After that, He would disappear, unlike His New Testament manifestation in flesh at Bethlehem.
Les explains from Colossians 1:15 that the pronoun who refers back to the Son in verse 13, the One in whom we have redemption and who has delivered us from the power of darkness and translated us into His kingdom. The Son is the image, the visible, tangible evidence, of the invisible God. The invisible God is the triune God: Father, Son, and Spirit, who are Spirit and were invisible. Out of that invisible triune God stepped the Son, and He became visible in the Old Testament economy temporarily, then He would disappear. In the New Testament account, He became visible, tangible, and touchable, living in the flesh by virtue of Bethlehem, the same God now manifested in the flesh by the Son.