The pasuk says in Tehillim, אם ה' לא יבנה בית שוא עמלו בוניו בו – if Hashem will not build the house, the builders will be toiling in vain. This pasuk was said by David HaMelech to his son Shlomo regarding the construction of the first Beit HaMikdash. And the lesson he gave him is extremely powerful. David was saying, you may have all the money in the world, all the manpower, the best architects, the top engineers, everything at your fingertips, but if you don't have Hashem helping, then all of that is completely worthless. The pasuk continues, אם ה' לא ישמר עיר, שווא שקד שומר - if Hashem will not guard the city, then all of the efforts of the guard will be in vain. David was saying, you may have the best possible army and the most powerful ammunition, but if you don't have Hashem guarding, then all of that is worthless.
How important it is for us to truly believe that we are nothing without Hashem. When the Jewish People were in jeopardy of going into exile, Hashem sent the navi Hoshea to tell them what to do to save themselves, and he gave them two instructions. First, make teshuva. And second, stop depending on the strength of man. He said, you can't rely on Ashur to help you, you can't rely on Mitzrayim to help you and don't think that your own strength can help you. You must know that you have nothing and are completely dependent on the mercy of Hashem.
The next pasuk says, if they will accomplish those two things, Hashem will have an outpouring of love towards them and all of the decrees would be abolished.
This is how valuable believing that people are nothing without Hashem is. The more a person believes he is dependent on Hashem, the more Heavenly mercy he gets. And the more he feels he can handle things on his own, the less mercy he gets.
Rav Aharon Rabinowitz of Lida, a small town in Lithuania, once visited a very wealthy man attempting to raise money for a certain cause. This man had an enormous factory that was used for grinding wheat. Everyone from his town, and all the neighboring towns, came to his factory to grind their wheat, and that alone made him wealthy. In addition, he owned hundreds of acres of land, as well as boats and forests. He told the Rabbi, he serves Hashem, but not out of fear because he has nothing to fear. He was fabulously wealthy, plus he had insurance on all of his properties and holdings.
However, one day, one of the top officials in the Russian government came riding by on his horse and he rode over the bridge that this wealthy man made by the river near his mill. The bridge collapsed and the official fell in and drowned. The wealthy man was accused of causing the death of this official on purpose and they claimed that he was a spy for Poland. He feared what the Russian authorities would do to him and so he took all of his money with him and ran away to the other side of Poland. He figured he would start a new business there with all of the money he brought with him. But to his dismay, a short time after he arrived, the currency that he brought with him was disqualified by the government and he was left poor and destitute. He was forced to sleep in a room in the local shul that was designated for the poor people of the town.
A couple of months later, Rabbi Rabinowitz came there and saw this man and couldn't believe he was living in such poverty. The man said to him, "Hashem showed me clearly that we could never rely on our own strengths and possessions."
We need Him every second of the day and no matter how much a person has, he is nothing without Hashem.