John Chrysostom
Paul calls God to witness not because they lacked trust in him, but because of his great disposition, and also for greater persuasion and encouragement.
Since he said that the Philippians were fellow partakers with him through almsgiving, he added, "with the love of Jesus Christ," εν σπλάγχνοις Χρίστου. He did this so that they would not think he loved them for that reason, and not simply for themselves.
What does this mean? It means according to Christ: because you are faithful, because you love Christ, because of love according to Christ.
He did not even say, "according to love," but something much stronger, "with the love of Jesus Christ." It is as if he said: I love you as your father, by kinship in Christ.
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This gives us a heart, σπλάγχνα, a warm and fiery heart. Such a heart is given only to His true servants. With such a heart, he says, I love you.
It is as if he said: I love you with a heart that is not fleshly, but most fiery, Christ's own. "That I love you all," he says.
I love all of you, because all of you also love me. How strongly I love you cannot be expressed. I cannot portray my love in words. I leave it to God, who penetrates the hearts, to see it.
The Apostle would not have called God to witness if he were flattering them, because that is not without danger.