The Big Bang theory was built from the mathematical model and ideas of a priest (Father Lemaitre) who happened to be a decent astronomer AND a fantastic mathematician.
When the theory came out in 1948, a famous astrophysicist (Fred Hoyle) was very much AGAINST it. Fred was a proud atheist - claimed it made him a better scientist. In 1949, he came up with his own theory (Steady State); it is during a radio interview in 1949 that he made up the nickname "Big Bang" for the other theory. As far as Fred was concerned, the Big Bang was just a ploy, by the priest, to give his God a role to play in the "creation" of the universe.
This awful nickname is too bad: it gives the false impression that the theory describes an explosion (it does not). The theory "simply" describes the effect of the expansion of space on the energy content of the universe:
Same amount of total energy + more space to spread it out =
the energy density goes down (= the universe cools as time goes on).
Steady State proposed that the expansion of space did NOT cause a lowering of the energy density, because new energy was constantly created, making the universe eternally the same.
Back then (in the early 1950s), most scientists - and most atheists as well - went for Steady State simply because the mathematics were a lot easier. The mathematics in Father Lemaitre's model were so complicated (tensors) that even Einstein had a hard time with the model (he finally accepted it).
We had to wait until 1964 for sufficiently advanced instruments (radiotelescopes) to tell the difference between the two. Steady State was dropped and Big Bang appears to be, for now, very useful (it is not 100% perfect).
In the past, the universe was hotter (this is now an observed fact). The energy density was higher and that allows for things to happen in "young" galaxies (billions of years ago) that are no longer going on in older galaxies.
The Big Bang theory (the real one) can only go back for a limited time (13.8 billion years). At that point, the energy density was so high that we no longer understand how things work. The theory is silent about whatever could have existed "before". Some people use the phrase "the Big Bang" to mean that moment (which we call the Planck Time or Planck Moment).
Father Lemaitre simply assumed that this initial moment is the moment of creation by [his] God.