The King James translation is great, but like any translation it is not perfect.
In the original Hebrew, the first part of the verse above reads as follows:
"Every weapon that is formed against thee shall not prosper."
There is something total, something complete, something very comforting and gratifying about this revelation.
There will be weapons that form, and there will be many in our lives that come after us.
But we have rock solid, rocksteady assurance, that every single one of those weapons will not prosper. Whatever aims, whatever the enemies, or the Enemy, may design against us, they will not succeed.
Consider these promises in God's Word:
"Whoso diggeth a pit shall fall therein: and he that rolleth a stone, it will return upon him." (Proverbs 26:27)
"And when David heard that Nabal was dead, he said, Blessed be the LORD, that hath pleaded the cause of my reproach from the hand of Nabal, and hath kept his servant from evil: for the LORD hath returned the wickedness of Nabal upon his own head." (1 Samuel 25:39)
Consider what Joseph said to his brothers when they begged for his forgiveness a second time:
"20But as for you, ye thought evil against me; but God meant it unto good, to bring to pass, as it is this day, to save much people alive." (Genesis 50:20)
God is on our side, and who can be against us? (Romans 8:31)
We are more than conquerors through Him, as well (Romans 8:37).