The essence of faith and the basis for understanding the unity of G‑d is to understand the applications for each Name. For all His Names that are mentioned in the Torah are included in the Tetragrammaton, Havayah, which is similar to a tree trunk. Each of the other Names - those which I have compared to roots and branches and other hidden treasures - has a unique function.
Each of the other Names...has a unique function.
It is just like a storehouse which has several rooms. Each room within the storehouse has a specific identity: one room has precious gems, one has silver, another has gold, while another has different kinds of food and another has drinks. If a person needs food, he may starve to death if he doesn't know how to get to the room, even though the rooms are full. It is not because his request has been denied. He is simply not aware of which room he needs.
So it is with the comprehension of the blessed Holy Names: there are Names in charge of prayer, mercy and forgiveness, while others are in charge of tears and sadness, injury and tribulations, sustenance and income, or heroism, loving-kindness and grace.
If one does not know how to concentrate on the very Name which is the key to the answer of his request, then who is to blame if the request is not granted? It is his own foolishness and ignorance, as it is written: "A man's folly subverts his ways, and his heart rages against G‑d." (Proverbs 19:3)
G‑d is open to everyone.
For G‑d is open to everyone. It is the foolishness of man that is to blame - like the one who does not know to which room he should go and therefore returns empty-handed. Yet, man thinks this evil thought, that G‑d thwarted him from getting what he needed. This, however, is not the truth, for his own foolishness has let him down. This is it is written: "It is your iniquities that have diverted these things, your sins have withheld the bounty from you." (Jeremiah 5:25)
One must therefore familiarize oneself with the ways of the Torah and know the purpose of the Holy Names. He should be expert in them, and when he needs to request something from G‑d, he should concentrate on the Name designated to handle that question. If he does so, then not only will his request be granted, but he will be loved in the heavens and beloved in the world; he will inherit both This World and the Next.
[From "Gates of Light - Sha'arei Orah", p.13 (Harper Collins)]