The word "prosper" means "successful, succeed or happen as expected". The LORD God declares to us in Isaiah 54:17, "No weapon that is formed against thee shall prosper; and every tongue that shall rise against thee in judgment thou shalt condemn. This is the heritage of the servants of the LORD, and their righteousness is of me, saith the LORD." God's Word is true! Then why do we experience attacks from the Enemy that seems to be successful or happen as expected? Here's the answer. Whenever we obey God's Word, it is ONLY then whatever weapon Satan forms against us will not succeed or happen as expected. But IF we are experiencing attacks from the Enemy and it's evident that his weapons have succeeded in our life, then it only means that we have given the Enemy permission. The permission is an open door. This open door ALWAYS stem from disobedience to God's Word because the LORD said that the soul (the mind, will and emotions of a person) that shall sin or disobey Him, it shall die (Ezekiel 18:20). The Enemy takes full advantage of this particular word when someone, especially a child of God, choose to disobey the LORD.
It also means that even God's servants, especially after ministering His Word, should not be experiencing bodily attacks such as falling downstairs, a broken arm or leg, scratches on their body, headaches as though someone is driving a nail in their head, stomach aches as though one has diarrhea, and choking as though someone's hands are around their neck. These are merely a few demonic manifestations from Satan's attacks. IF demonic manifestations are evident in the life of a Believer, then it only means that the individual has an opened door in which they have given the Enemy permission to be successful in using his weapons in their life. In a nutshell, when we have obeyed the voice of the Holy Spirit in every area that He has revealed to us and we continue to be led and guided by Him, then with all surety we KNOW that the Word of God is true in which no weapon that the Enemy has formed against us is successful and will not happened as expected. Selah †