Yea, it contain spoilers.
Mother and Father had a boy called Isaac. Mother wanted a girl, so she dresses Isaac like one. Father didn't liked that, so he discussed with Mother. Mother was angry, and she thought that all was Isaac's fault, and cuz of that she took a knife and tried to kill little Isaac, but Father prevented it. After that Father left home. Mother was crying, a lot. Isaac felt guilty, so he found a chest in her room, he locked inside, and wait until his dead.
The Hush is you fighting against the dead, in other words, you being suffocated. Hush scales with your damage, more strong you are, more strong he is, because he is you. Delirium is an amalgam of all the bosses that you fought, and it is the last step before die, you remembering all your life.
There are other thing curious, all the monsters that you fight are biblical references, like Satan or the angels; Isaac's fears, like spiders or eyes; and how Isaac looks himself.
Other fact is that all playable characters are also how Isaac looks himself, like Judas, who betrayed Jesus; Eve, who committed the first sin; Magdalene, the girl that Mother wanted...
And the most shocking fact, if you beat the game, Isaac dies. There are two possibles finales.
The first one is Isaac going to the heaven or the hell and after that, to the Chest. In this final, Isaac fights against himself and Satan. After beating them, he admit his guilty and locks up in the Chest.
The second one in apparently after you get the first final, cuz Isaac is going to face against Delirium, and after wining the match, Isaac with die.