God, who was invisible, had become visible and had been seen by his people in various ways (anthropomorphic visions, angelic visits) in sundry times in the Old Testament. All appearances of God in the Old Testament was related to Jesus Christ, the image of the invisible God.
In the New Testament, it was revealed that only Jesus Christ had seen the invisible God (cf. John 1:18, 6:46). Seeing the Father means ability to do the same things as the Father "The Son does whatever he sees the Father is doing (John 5:19)." Here the action of the Father is being imitated by the only begotten Son who sees exactly what his Father is doing and this relationship shows the Son is omnipotent and of the same nature with the Father whom he can see and imitate. The prologue of John tells us that Jesus Christ in his pre incarnate state was both God and through whom all things came into being (John 1:1, 1:3).
In the Book of Revelation, Jesus Christ himself sent his message through an angel. Thus, the angel stands in the place of Jesus Christ as a representative. This might be the case in the many instances of God appearing to men in the form of a man and was called an angel in the Old Testament. They had seen the shaliach (agents) of the Word who was both God and only begotten God (John 1:1, 1:18).
It is also noteworthy that the Father is depicted as unseeable in the book of Revelation and that only Jesus was visible (as the Lamb) on the throne of the Father.
Justin Martyr in the second century C.E. spoke of the angel who appeared to ancient Israel in the form of a man to be the pre incarnate Jesus Christ himself. Yet Justin also explained that it was not Jesus himself but the one who has the name "Jesus" in him, quoting Exodus 23:21. Hence, the angel who claimed to be God in the form of a man was representing God the Son, Jesus Christ.
On the other hand, God appearing in anthropomorphic visions like in Isaiah 6 was deemed to be the pre-incarnate vision of Jesus Christ himself according to John. Isaiah saw his [Christ's] glory.
In the Gospels, Jesus Christ himself acknowledged that he was the "one who is like the son of man" in Daniel 7. In the Old Greek (OG) of Daniel 7:14, the son of man was described as "coming as the ancient of days" which identified the son of man as the LORD God himself. If this were the case, Jesus Christ must also be the LORD God who appeared to Ezekiel in the form of a man (Ezekiel 1:26).
Discussion
The angels of Jesus Christ , the only begotten God, who appeared representing him in the form of men on his behalf in the Old Testament and in the book of Revelation should not be confused as the same person as him. Jesus Christ having a Shaliach angel shows that Jesus was the God of gods, the Most High, like the Father.
We do have many clear instances of Jesus Christ himself appearing as a man (theophany) in visions in the Old Testament. In these visions, Christ himself in glory appears in the form of a man. In the Old Testament, visions of the LORD God (YHWH) were visions of the Lord Jesus Christ.
Conclusion
God the Father appeared neither in the Old Testament nor in the New Testament. God the Father never took on a visible form either through emissaries or visions or incarnation. Only God the Son appeared visibly in both testaments and he appeared various ways in sundry times with the incarnation as his final and permanent theophany.
- Christ sends an angel as his emissary and had his name in him (Exodus 23:21).
- Christ appears in visions (in the form of a man or a lamb etc.).
- Christ took on flesh and revealed himself in the flesh at the incarnation
Note that God the Holy Spirit himself appeared , at least once, in the bodily form (ειδος) of a dove at the baptism of Christ. This is another theophany.