This rejoicing [in Philippians 4:4-9] is not separable from grief, for indeed it is rather deeply connected with grief. The one who grieves for his own wrongdoing and confesses it is joyful. Alternatively it is possible to grieve for one’s own sins but rejoice in Christ. On this account Paul says “rejoice in the Lord.” For this is nothing if you have received a life worthy of rejoicing.

–John Chrysostom, Homily on Philippians 15.4.5-7