Rationalism is demanded by Atheism. It is defined as reaching conclusions by using only logical, demonstrably true, and perhaps even empirically valid, premises. A rational conclusion is one that is derived because it is valid.
Rationalization is the stacking of premises toward the proof of a previously established conclusion. A rationalized conclusion is one that is desired to be valid, regardless of whether or not it actually is valid.
For rationalizing a pre-defined conclusion, only favorable premises are used to support the argument. So Rationalization is a fallacy, because it cannot follow that the conclusion is actually valid when it is possible that negative necessary premises have been ignored. Rationalization is actually an emotional process, where rationality is strictly intellectual.
Example: Empiricism has been defined as a naturalistic process because it is necessary to physically observe and validate the premises. Within Empiricism, then, Naturalism is a necessary condition. However, this condition has been extrapolated to mean more than a condition; it has now become a conclusion in the following sense:
Because empiricism can see nothing beyond the physical realm, then the physical realm is all that exists.
In this statement, the roles of Empiricism and Naturalism have been reversed. Empiricism, once the conclusion determined by the premise, Naturalism, has now become the premise of the conclusion, which is Naturalism.
In other words, Naturalism changed from a condition to a conclusion. So Naturalism changed from a necessary (but not sufficient) condition for Empiricism, to a necessary and sufficient conclusion for a worldview. Because the conclusion is based upon an exclusionary principle, with other philosophically valid premises ignored, or denied necessity, it is a rationalized conclusion, and therefore based on fallacy.
This role reversal requires that many of the premises which are ordinarily considered philosophically intuitive also be rationalized in order to fit them to the preconceived conclusion. Thus transcendent functions such as love, altruism, ethics, self-awareness, consciousness, abstract thought, personal relationships, rebellion, reversal of previous thought processes, and on and on…. are all rationalized into being physical entities: they have to be physical, because they cannot - by definition - be non-physical. So they are pre-defined by the desired conclusion, rather than the conclusion being based upon objective examination of the premises.
This is an actual statement made by an evolutionist. If we are here, then we evolved, because the conclusion, evolution, is assumed to be true. Evolution is now assumed, and is a condition for interpreting virtually all aspects of living, and life, from genetic and organic evolution, to psychological and ethical evolution. The proof for evolution is discussed elsewhere, but it is a vast collection of circumstantial evidence, much of it frivolously extrapolated to provide rationalized premises to support the pre-established conclusion. This is known as Post Hoc Fallacy, aka "Just So Stories" (after Kipling). And circumstantial evidence, even in vast amounts, is never absolutely conclusive. It is always probabilistic.
Is it possible to avoid forming a worldview based on probabilistic information? Yes. Is it possible to avoid forming a conclusion based upon a worldview? Yes again. But it is difficult if the worldview is biased toward self gratification.
A rational form of the rationalized statement above might look something like this: If evolution is true, and if it applies to us, and if there are no transcendence, no singularities, and no initiating first cause of the universe and first life, then there is a certain probability of our having evolved. Some of these conditional premises are not knowable, period. So the probability of evolution being a valid conclusion, must less a valid premise, is absolutely unknown.
When Rebellion occurs, inversion of the logical process also occurs, and much of what passes for logic in the state of rebellion is rationalized rather than rational, in order to justify the rebellion, and the resulting hedonism. So rebellion tends to feed itself, by stacking up rationalized premises in its favor.