As far as I am aware, Christians don’t ACTUALLY believe that prayer is effective. Despite the promises of Matthew 17:20 or John 14:12-14, Christians know that their prayer and their God is toothless for the things that prayer would really be effective for.
Show me the multitudes of children who were terminally ill with brain cancer – got prayed over – then had miraculous spontaneous recoveries. Show me all of the teenage para and quadriplegics who stood up and walked upon receiving prayer. Show me the sex predators who were restrained by God just as they were about to harm an abducted child. Show me the hundreds and hundreds of missiles that were inexplicably deflected just prior to leveling apartment buildings, hospitals, and civilian bomb shelters in Ukraine.
Christians will give you a dozen examples of where prayer (and their God) made a difference when it comes to events that were either inevitable or events that had a moderately unlikely chance of occurring: little Billy’s appendices surgery was successful… I found a $100 bill on the street when I had no money for rent… I was having a terrible day and “just the right song” came on the radio after I prayed for peace. But examples where God having a truly miraculous impact? I think the results speak for themselves.
I roll my eyes now when I think about the story that the biblical writers talk about Jesus in the desert, and the part in Luke 4:12 or Matthew 4:7 where Jesus says: "Do not put the Lord your God to the test.” There is a good reason why you should not test God! It’s because he fails any real substantive difficult test, every single time.