Note 5 at Acts 21:32: It was certainly not Paul's time to die as can be seen by the Lord's later appearance to him and instruction that he was to also bear witness of Jesus in Rome (Acts 23:11). Therefore, the Lord had to deliver Paul from this attempt to kill him.

There were a number of times when Jesus had just supernaturally walked through the midst of His persecutors (Lk. 4:30; Jn. 7:30; 8:59) and this could have happened with Paul. Yet the Lord used the Roman military in this case to protect Paul. In Lystra and Derbe, the Lord didn't deliver Paul from stoning, but rather supernaturally raised him up after the mob had left him for dead (see note 3 at Acts 14:20, p. 658). We can be assured of the Lord's protection but we cannot be sure of the method that the Lord will use.