Therefore I say unto you, What things soever ye desire, when ye pray, believe that ye receive them, and ye shall have them. – Mark 11:24

When I was conversing lately with our dear friend George Miller, he frequently astonished me with the way in which he mentioned that he had for so many months and years asked for such and such a mercy, and praised the Lord for it. He praised the Lord for it as though he had actually obtained it. Even in praying for the conversion of a person, as soon as he had begun to intercede he began also to praise God for the conversion of that person. Though I think he told us he had in one instance already prayed for thirty years and the work was not yet done, yet all the while he had gone on thanking God, because he knew the prayer would be answered. He believed that he had his petition, and commenced to magnify the Giver of it. Is this unreasonable? How often do we antedate our gratitude among the sons of men! If you were to promise some poor person that you would pay his rent when it came due, he would thank you directly, though not a farthing had left your pocket. We have enough faith in our fellow-men to thank them beforehand, and surely we may do the same with our Lord. Shall we not be willing to trust God for a few months ahead, ay, and for years beforehand, if His wisdom bids us wait. This is the way to win with Him. When ye pray, believe that ye receive the boons ye ask, and ye shall have them. “Believe that ye have it,” says the Scripture, “and ye shall have it.” As a man’s note of hand stands for the money, so let God’s promise be accounted as the performance. Shall not heaven’s bank-notes pass as cash? Yea, verily, they shall have unquestioned currency among believers. We will bless the Lord for giving us what we have sought, since our having it is a matter of absolute certainty. We shall never thank God by faith and then find that we were befooled. He has said, “All things whatsoever ye shall ask in prayer, believing, ye shall receive.” And therefore we may rest assured that the thanksgiving of faith shall never bring shame into the face of the man who offers it.~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/1469.cfm