How could Paul rejoice when his adversaries were constantly opposing him?
Paul could rejoice because joy is distinct from happiness. Though his adversaries constantly gave him a bad time and his circumstances were not happy, he still had joy. His confidence that he would abide and continue with the Philippians for their furtherance and joy of faith sustained him.
Les explains that when things are all going wrong, it is hard to be happy, but a person can still have joy. Philippians is packed with the concept of joy and rejoicing. In Philippians 1:18, the verses just ahead describe Paul's adversaries giving him a bad time constantly. That was not a place of happiness for Paul, but he had joy, so he could rejoice even though he had all that opposition. Then Philippians 1:25 gives his confidence: “having this confidence, I know that I shall abide and continue with you all for your furtherance, and joy of faith.” The Philippian believers were not living in a happy environment either; they were under constant pressure. Paul's rejoicing was therefore not based on pleasant circumstances but on his confidence and his concern for their furtherance and joy of faith.
What then? notwithstanding, every way, whether in pretence, or in truth, Christ is preached; and I therein do rejoice, yea, and will rejoice.