There are lots of ministers that will speak to an issue, or even command a healing or deliverance in the name of Jesus. Some teach everyone can "call things into being what does not yet exist." (Rom 4:17) Also, II Cor 4:13 (niv) It is written: “I believed; therefore I have spoken.” Since we have that same spirit of faith, we also believe and therefore speak,
So your pastor was aiming quite high to declare your healing. Unfortunately, it did not work at least instantly. I have seem much of this in my lifetime. Some do receive, so I would not blame the pastor entirely, at least he was trying. The big question is: Are the results up to God, or are the results up to our faith? Certainly any minister can speak the words, but not all ministers and people receiving, believe them. The late Dr. Fred Price wrote a great book on this.
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Some have faith, others presume and others are just foolish. Still, I would not reason that it is God's desire to keep your diverticulitis. But use it as a time to learn from God how to get the deep faith to overcome the affliction. Sure you can find a pastor that will agree that perhaps it is a thorn in the flesh or something God wants you to carry. But Jesus is the same yesterday, today and forever. He went about healing all, except in his home town he healed few because of their lack of faith. One man asked if Jesus if he were willing, another asked him to help his unbelief. Jesus helped them both. Jesus too marveled at the Centurion's faith who said Jesus could just speak the word and it was done. Rather than reject the pastor, why not go to him and let him know you are still suffering? I knew a man in my church, God bless him, that went up front dozens of times for a specific healing. I moved so I do not know what happened with him but I like that he and the pastor kept knocking. You too have a part in believing for a healing. When you are saved you confess with your mouth and believe in your heart. You have to find that belief part. Yes, that you know that God is for you and that he has done it past tense.
I will say too that nearly all the ministers that teach this type of faith will go to a MD if needed. Yes, God too heals and relieves suffering through doctors.
Of course there are critics of this type of teaching but I just thought I might help you understand it better. Typically it is called the word of faith type teaching. Critics may call it the name and claim it, but alert adherents know its limitations, particularly that the faith has to be real, not manufactured. Real faith for anything comes from God and hearing his word. Hope this helps