2 Corinthians 8:9

For ye know the grace of our Lord Jesus Christ, that, though he was rich, yet for your sakes he became poor, that ye through his poverty might become rich.

Though he was rich ... Adam Clarke's perceptive comment on this should be remembered. He said:

If Jesus Christ was only a man, in what sense could he be rich? Joseph and Mary were poor in Jerusalem, and poor in Nazareth; and, from the stable to the cross, Jesus never possessed any property among men, nor did he have anything at his death to bequeath, except his peace! The question of the riches of Christ, on the Socinian scheme, can never be satisfactorily answered.[15]

The riches of Christ are those riches which pertained to his status with God and equality to God before the world was (John 17:5), the riches of His eternal power and Godhead, the riches of His everlasting divinity and glory. Only such an explanation as this can pertain to Paul's words here.

He became poor ... Christ's becoming poor has a double meaning, (1) referring to the contrast between his eternal state and his incarnation, and (2) also to the extraordinary poverty of his earthly state as compared with the affluence of some of his contemporaries.

For your sakes ... It should ever be remembered that Christ forsook heaven with its glory to live upon earth with its shame in order to redeem men from the curse of sin. It was not merely for the sake of the Corinthians, but for the sake of every man, that he thus "humbled himself" and took upon him the form of a servant, and was found obedient, even to the death on the cross!

As Hughes said:

Paul felt none of the embarrassment which is displayed by some modern scholars who, because of a preconceived antipathy to "supernaturalism," would prefer to dismiss this doctrine of Christ's pre-existence.[16]

The simple, objective truth of Christianity is founded upon the conviction of the supernatural. In the final analysis, if there is no supernatural, there is no Christianity. So-called Christians who do not believe in the supernatural are unbelievers; and there can be no reconciliation of the supernaturalness of Christianity with the existential and speculative denials of it. What is affirmed in the New Testament is either true or false; and this student of the New Testament believes it to be true. Paul here assumed as fact, nor did he even pause to defend it, that Christ existed with God before the earth was created. No one can know the mind of Paul without seeing this fundamental truth.

[15] Adam Clarke, Commentary on the Holy Bible (New York: Carlton and Porter, 1829), Vol. VI, p. 349.

[16] Philip E. Hughes, op. cit., p. 301.