Behind the wall
Man was originally created to live in a garden paradise (Gen 1-3). There was no danger there except man's own potential to sin. Because man sinned he lost that paradise and was driven out. As the death spiral of sin continued to take man further away from God the world became increasingly dangerous. First Cain killed his brother Abel (Gen 4), and then violence began to take over until God had to send a flood and destroy the world (Gen 6-8). After the flood, man was to spread out and repopulate the earth, but he chose instead to form the first big city (Gen 11). It was called Babel, or as it was known later, Babylon. There are advantages to living in large population centers, but there are also serious problems that always arise due to the tainted nature of man. Most cities are built after the same spirit of Babylon which is "let us make us a name" and in so many words "let us build our own protection". Since man by and large does not obey the word of God his faith is not in the protection of God, so he goes about trying to protect himself. There is nothing wrong with taking common sense measures, but when we leave God out of our preparations they are bound to come up short.
So men began building walls around their cities to protect themselves from other men. We see walled cities throughout the history of the Bible. Even new Jerusalem will have walls and gates (Rev 21-22). There are some particularly notable walled cities such as Jerusalem (the most important important city in the Bible), Jericho whose walls famously fell down (Josh 6), and Babylon. They would set watchmen on the wall of the city to keep an eye out for potential danger. However, as the psalmist said, "Except the LORD build the house, they labour in vain that build it: except the LORD keep the city, the watchman waketh but in vain (Ps 127:1)." The peole of Jericho learned this when their wall fell down and the Israelites marched straight into their city. The people of Babylon also learned this when the Persians fulfilled the prophecy of Daniel the very night of his prophecy by blocking the Euphrates river and entering and conquering Babylon (Dan 5).
Man has not learned from history. He still thinks he can build walls God cannot get behind. Nations do this. Religions do this. Individuals do this. China built its famous wall, and later thought their atheistic communism could keep God out of their nation. China had an underground church larger than the church in free America. The Muslims think they can set up their regimes based on sharia law and keep the God of the Bible out of their countries. Individuals think they can build walls around their hearts that God Himself cannot go behind. Yet God continually shows us man cannot build a wall too big for God to get behind. God is revealing Himself and pouring out His Spirit in places men think they have Him walled out of.
People get hurt and they become distrusting of others and defensive. They put up walls to protect themselves. This is understandable and predictable. Jesus said, "And because iniquity shall abound, the love of many shall wax cold (Mt 24:12)." While it may be difficult to trust some people, and others should not be trusted, we must never put up a wall to keep God out. He has never hurt us or done us wrong. The sins of man are not God's fault. He is the only One we can fully trust because He loves us with the purest, fullest, most unconditional, most faithful love there is. We are only perpetuating the pain He wants to heal when we do not let Him behind our walls. He will not violate our will. He waits to be invited. "Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me (Rev 3:20)." We may struggle with trust, but God is bigger than any wall. He can help us overcome. We may have to march around our Jericho a few times until the wall falls, but we can learn to trust God. Then He will help us to learn to trust the right people.