The rock is the metaphor for the brain, and the sword is the symbol for the tongue, which is the double edge sword (word), i.e. can either aid or do harm, that is rooted in the brain, hopefully, aka in listening, same way as, metaphorically speaking, when St Peter gets this name by becoming the gate keeper of perception archetype, aka the pineal gland, sorting out what the conscious mind, aka the Christ, should be made aware of or not from the multitude of information present in the subconsciousness at any moment, thus right before that moment St. Peter is known as Simon, which means to hear in Hebrew. Therefore the image tells us: listen and think before speaking, for a word is not a bird, once it got out one can't catch it.