after John was arrested

"after John was placed in prison." Matthew is referring to when King Herod had John arrested. This can be stated in active form. Alternate translation: "after King Herod had John arrested" or "after soldiers arrested John" (See: rc://en/ta/man/translate/figs-explicit and rc://en/ta/man/translate/figs-activepassive)

proclaiming the gospel

"telling many people about the good news"