Violence shall no more be heard in thy land, wasting nor destruction within thy borders; but thou shalt call thy walls Salvation, and thy gates Praise. (Isaiah 60:18)The bit about calling our walls Salvation and our gates Praise sounds really pretty, but I think Isaiah is trying to teach us a spiritual lesson here.
Just like walls surrounding a city keep the enemy out, when we have repented of our sins and obtained the change of heart through Christ’s Atonement so that we have no more desire to evil, the abhorrence of evil becomes a barrier between us and sin that keeps us safe from it. It is a wall made of salvation.
Once we have this defensive wall against sin, how do we decide to let things into our lives? A walled city can decide to not let anything or anyone into the city unless it or they have been praised as something or someone good. In this way, the requirement of praise can be like a filtering gate. We seek after things that are praiseworthy. And of course it has to be praised by someone we trust.
So how effective are these defenses? We find out in the Book of Mormon:
3 Behold, I said that the city of Ammonihah had been rebuilt. I say unto you, yea, that it was in part rebuilt; and because the Lamanites had destroyed it once because of the iniquity of the people, they supposed that it would again become an easy prey for them.Image credit: Best of Edinburgh, http://www.bestofedinburgh.com/Page.asp?Title=Edinburgh+Castle&Section=25&Page=1.
4 But behold, how great was their disappointment; for behold, the Nephites had dug up a ridge of earth round about them, which was so high that the Lamanites could not cast their stones and their arrows at them that they might take effect, neither could they come upon them save it was by their place of entrance.
5 Now at this time the chief captains of the Lamanites were astonished exceedingly, because of the wisdom of the Nephites in preparing their places of security. (Alma 49:3-5)