Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church: Rejoice in my sufferings for you - St. Paul always considers his persecutions, as far as the Jews were concerned in them, as arising from this simple circumstance - his asserting that God had chosen the Gentiles, and called them to enjoy the very same privileges with the Jews, and to constitute one Church with them.
It was on this account that the Jews attempted his life at Jerusalem, when, in order to save it, he was obliged to appeal to Caesar; the consequences of which persecution he was now suffering in his imprisonment in Rome. See on Colossians 4:2 (note).
That which is behind of the afflictions of Christ - I have still some afflictions to pass through before my race of glory be finished; afflictions which fall on me on account of the Gospel; such as Christ bore from the same persecuting people.
It is worthy of remark that the apostle does not say παθηματα, the passion of Christ, but simply θλιψεις, the afflictions; such as are common to all good men who bear a testimony against the ways and fashions of a wicked world. In these the apostle had his share, in the passion of Christ he could have none. He trod the wine press alone, of the people there were none with him.
His body's sake - Believers, both of Jews and Gentiles, who form that one body, of which Christ is the head.