When was he rich?

"For you know the grace of our Lord Jesus Christ, that though he was rich, yet for your sake he became poor, so that you by his poverty might become rich."

2 Corinthians 8:9

With the ghost of Adolf Von Harnack looking over his shoulder and nodding with approval, Hans Kung describes the idea of the incarnation as a Hellenistic concept, conjured up by Hellenistic councils, deriving from a Hellenistic world. 

Kung writes:

"In the context of the history of the Jew Jesus, the Greek conceptual model of 'incarnation' must to some degree be buried." (Credo, p. 60-61).

Along with this withdrawal from the incarnation comes a denial and re-interpretation of the pre-existence of the Son of God.

But Paul says that 'though he was rich' Christ became poor.  And as an old Welsh preacher put it "He became poor when he came to Bethlehem.  Tell me, when was he rich?"

Or as Maxentius expressed it against those who deny the eternal pre-existence of the Son: 

We do not confound the diversity of natures, howbeit we believe not what you affirm, that Christ was made God; but we believe that God was made Christ.  For he was not made rich when he was poor; but being rich, he was made poor, that he might make us rich.

He did not take the form of God when he was in the form of a servant; but being in the form of God, he took on him the form of a servant.  In like manner, he was not made the Word when he was flesh; but being the Word, he was made flesh.