Albert Barnes on Philippians 1:8

For God is my record. My witness; I can solemnly appeal to him.

How greatly I long after you all. To see you; and how much I desire your welfare.

In the bowels of Jesus Christ. The word "bowels," in the Scriptures, denotes the upper viscera--the region of the heart and lungs. Isa 16:11. That region was regarded as the seat of affection, sympathy, and compassion, as the heart is with us. The allusion here is to the sympathy, tenderness, and love of the Redeemer; and probably the meaning is, that Paul regarded them with something of the affection which the Lord Jesus had for them. This was the tenderest and strongest expression which he could find to denote the ardour of his attachment.

(+) "bowels of Jesus Christ" "With the tender affection of"