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 be rich.
Ye know the grace - the gratuitous love whereby the Lord emptied Himself of His heavenly glory (Philippians 2:6-7) for your sakes.
Became poor. Yet this is not demanded of you (2 Corinthians 8:14), but merely that, without impoverishing yourselves, you relieve others with your abundant. If the Lord did so much more, at such a cost, for you; much more may you do an act of love to your brethren at so little a sacrifice. Olshausen, 'As Christ by becoming poor made you rich, ye can thus bestow of your abundance upon others, for to this end ye were placed in this condition.'
Might be rich - in heavenly glory, and in all other things that are really good for us (cf. 1 Corinthians 3:21-22).