The very idea of language must be acquired. In this sense, religion is like language. No particular language is innate, although all humans normally do acquire one and have a capacity (some say an instinct) to acquire one. . . . Which actual language a human acquires depends on his or her social environment.
A person acquires a particular language simply by and because of being raised among others who already speak it; generally, people speak whatever language is spoken around them. . . . no speakers of a particular language would ever claim that their language is "true." The very notion that, for instance, English is true while Spanish is false is nonsensical.
Interestingly, religion is like language in a number or ways--none of them fortunate for those who want to take religion seriously. First, while most humans end up believing some religion, no particular religion is inborn or natural. Therefore and second, there are very many particular real or potential religions . . . . Third, a person normally acquires religion simply by and because of being raised among others who already do speak it; and generally, people practice whatever religion is practiced around them. The profound difference between language and religion, however, is that all members of a religion think that their religion is true. The very notion that, for instance, a person would say, "I am a Christian, but I don't think Christianity is true" is contradictory and ridiculous (pp. 37-38).
Below I would like to list both comparisons and contrasts between religion and language, some of them from Eller, and some from my own thinking.
Points of comparison:
1. Religion like language is acquired.
2. People are born with a capacity or an instinct to acquire both language and religion.
3. People acquire whatever language or religion they are exposed to as a child.
4. People are better able to function in their culture by adopting the language of the culture and by adopting the religion of the culture.
5. Both language and religion acquisition is advantageous for a person who desires to "fit-in" in their social setting.
6. A person may acquire a second or more languages over the course of a lifetime.
7. In the evolution of language, words and grammatical forms are borrowed from one language and incorporated into another in a type of cross-fertilization. The same phenomena is seen in the evolution of religions.
Points of contrast:
1. Religions are thought to be true, whereas languages are not. A person may think of their language as "true" if it allows them to adequately communicate with other people in their society but they do not conceive of their language as being the only "true" language. If they are aware of other languages, they would acknowledge that those languages are "true" for the people in other cultures.
2. When a person acquires a new language, he does not then reject his old language.
While the analogy is certainly not perfect, I think it is useful in understanding how religion is a cultural phenomena.