Note that Charles and Cora say that "words are instruments of ideas." Behind every word is an idea. A word is an expression of an idea. The Fillmores are saying that while we have no real control over the ideas that flow into our mind, we can use words as instruments to guide them once they are present. In other words, we can kill the expresson of a divine idea with negative words and we can "enlarge and establish" the expression of a divine idea with "words of praise and blessing."
Note that Charles and Cora say that "words are instruments of ideas." Behind every word is an idea. A word is an expression of an idea. The Fillmores are saying that while we have no real control over the ideas that flow into our mind, we can use words as instruments to guide them once they are present. In other words, we can kill the expresson of a divine idea with negative words and we can "enlarge and establish" the expression of a divine idea with "words of praise and blessing."