The word Jesus used that is translated “keeps” in this exclamation is a verb: an action word. The noun form of it: thing, place word, is sometimes translated “prison.”
How tight is our grip on the things that Jesus has said and is saying? How possible is it to talk us out of believing them, like the serpent did Adam and Eve? How skilled have we become at defending them, like Paul did before Agrippa? There are many other promises to those who “keep” His words.
Bind them in reality to your heart and mind like the Pharisees did symbolically. In the law He gave Moses, Yahweh instructed the Israelis to write laws down and fasten them to their arms and foreheads. The Pharisees made leather boxes they called “phylacteries” to hold Yahweh’s words. If they had held them in their minds and hearts, they may have been able to take the next step under the guidance of the Holy Spirit, and recognized His Word when He stood before them speaking Life to them.