John Calvin Commentary Philippians 1:8

"For God is my witness, how I long after you all in the tender mercies of Christ Jesus." — Philippians 1:8 (ASV)

For God is my witness. He now declares his affection for them more explicitly, and, to give proof of it, he uses an oath—and for good reason, because we know how dear the edification of His church is in God's sight. It was also particularly beneficial that Paul’s affection should be thoroughly known by the Philippians. For it greatly helps to secure acceptance for the doctrine when the people are persuaded that they are beloved by their teacher.

He calls God as a witness to the truth, because He alone is the Truth, and as a witness of his affection, because He alone is the searcher of hearts. In the word translated long after, a specific term is used instead of a general one, and it is a sign of affection, because we long after those things that are dear to us.

In the bowels He contrasts the bowels of Christ with carnal affection, to indicate that his affection is holy and pious. For the person who loves according to the flesh looks to his own advantage, and may change his mind from time to time according to varying circumstances and times.

Meanwhile, He instructs us by what rule the affections of believers should be regulated, so that, renouncing their own will, they may allow Christ to sit at the helm. And, undoubtedly, true love can flow from no other source than from the bowels of Christ, and this, like a goad, should affect us significantly—that Christ, in a way, opens His bowels, so that through them He may cherish mutual affection between us.