Where does it say in the Old Testament that God will appear on Earth as a human?
You will find 44 references the messiah in Genesis, Exodus, Isaiah, Numbers, Micah, 2 Samuel, Daniel, Psalms, Hosea, Deut., Malachi, and Zecheria. As well as references below from the NT writers who knew of those prophesies and witnessed their fulfillment.
Acts 13:23
Of this man's seed hath God according to his
promise raised unto Israel a Saviour, Jesus:
Acts 13:32
And we declare unto you glad tidings, how that the
promise which was made unto the fathers,
Acts 26:6
And now I stand and am judged for the hope of the
promise made of God, unto our fathers:
Acts 26:22 Having therefore obtained help of God, I continue unto this day, witnessing both to small and great, saying none other things than
those which the prophets and Moses did say should come:
23 That Christ should suffer, and that he should be the first that should rise from the dead, and should shew light unto the people, and to the Gentiles.
Romans 1:2
(Which he had
promised afore by his prophets in the holy scriptures,)
Romans 15:8
Now I say that Jesus Christ was a minister of the circumcision for the truth of God, to confirm the
promises made unto the fathers: