We could also ask the opposite :
Are we defined as a atheist by birth ? Or are we required to decide on our own our atheism based on our own developed adult brain ?
Are we "born atheists" , as some people claim?
It is certainly true that babies are born without any specific religious faith, but does that make them "born atheists"? I do not think so.
First: Not every being that does not belief in God is an atheist. Nobody would argue that insects or sharks or mice are atheists, although none of them believes in God.
I'd say that we can only say "A is an atheist" if A is (in principle) able to grasp intellectually the concept of "God" and then decides not to subscribe to this concept.
Second: If we can say anything about babies and their relationship to religion, it is analogue to babies and their relationship to language. Babies cannot speak, but that does not mean that they are "a-lingual". Just the contrary: babies are born with a brain that is prepared to learn any language quite effortlessly. Therefore although infants cannot (yet) speak they are "born speakers"; all they need is sufficient linguistic input.
The same can be said - mutatis mutandis - about babies and belief (religious or not). They do not (yet) hold any belief, but they are born believers, as they are equipped with cognitive mechanisms to acquire any belief effortlessly. No toddler or child will ever come up spontaneously with the idea of the Christian god or the idea of karma or reincarnation, but s/he will adopt these beliefs easily because evolution prepared the human brain to expect and adopt beliefs like these.
For comparison: Evolution did not prepare human brains to learn how to write or to read or to do math, which is why it takes many years of intensive training to master these skills.
Given that Homo sapiens is a cultural animal, and given that culture (in addition to artifacts and knowledge about the natural world) also consists of fictions, tales, belief systems, imagined orders etc... , it is absolutely necessary that babies acquire and adopt the beliefs of their specific culture as effortlessly as their language.
The "gullibility" of toddlers and children is not a bug, it is a feature, the very characteristic trait that enables culture to exist and to be reproduced. It is this trait that makes us human.