In the nature vs. nurture debate very rarely can you completely control for one or the other. Monozygotic (identical) twins allows science to completely control for the nature portion because they have the exact same genetic code.
So,the remainder of the behavior would be a function of nurture, right? Not so fast, because twins are raised in the same environment. So, finding a pair of identical twins (same nature) raised in a different environment (different nurture) would allow us to draw the conclusion that similarities of behavior would be a function of the similarities of nature (in this case the identical genome). The environment (they grew up in different homes) would, in this case not be a cause of similarity at all since it is completely different.
Straight causation would need a 100% similarity of behavior rate ( in this case, religiosity). In this case, there was a 62% similarity of religiosity behavior. Is this significant? Yes, because no correlation at all would be 0%, and this is very nearly the statistic found (2%) in the case of fraternal twins raised apart. They had different nature (genes) and nurture ( environment.
So, we can conclude that there is a 62% correlation between genes and religiosity. Statistically speaking, this is a strong positive correlation.
However, this leaves room for other factors as well.
This is why the natural experiment of identical twins raised apart gives us tremendous insight into the relative amounts of nature and nurture that cause certain human behaviors.