As scientists study the tremendous diversity of living things, they classify them into large families and various smaller groupings based on the similarities and relationships they discover among them. This system of classification is called taxonomy — from the Greek words taxis or arrangement and nomos or law.

With the passing of time and growth in scientific knowledge, the classifications become more sophisticated and less obvious.

For example, once dolphins, along with large fish, were classified as sea creatures based on their obvious similarity of living in the sea. Now, even schoolchildren know the difference between sea creatures that breathe with lungs and those that breathe with gills. Dolphins are mammals, not fish.

Classifying dolphins — and seals, whales and walruses — as mammals is based on more than how they breathe. They also share a similar internal structure. That’s why bats are mammals and birds are not — even though they both have lungs and both fly.

Recently I took some visitors to the natural history museum. In the halls with enormous, towering skeletons of dinosaurs, an exhibit showing their relationships pointed out that their nearest living relatives are, surprisingly, birds.

Uncovering relationships like those between dinosaurs and birds means going much deeper in the search for similarities and commonalities, even to the level of DNA and genes.

Relationships at the genetic level upset a lot of common notions. For example, human beings are often classified into “races,” based on a superficial difference, the degree of skin coloration. At the deeper level, there are no basic differences among people at all. Also, surprisingly to some, present scholarship suggests East Africa as the likely place for human origins.

It is challenging to apply some of the same types of analysis to the classification of religions.

For example, Orthodox Judaism and Roman Catholicism seem very different. Yet, the crucified Christ who has such a prominent place in Catholic piety and theology can only be fully understood in terms of the various sacrifices — Passover, atonement, sin offering and thanksgiving — of the law of Moses, or the Torah, the heart of Judaism.

The origins of all forms of Christianity lie in Judaism. Early Christians were devout Jews who recognized Jesus of Nazareth as the hoped-for Messiah — in fact the very name Christian means “Messianist.” As time passed, they accepted non-Jews into their company. For Christians, this was a positive development. For Jews, this was a radical break and an abandonment of critically important values.

In this sense, a commonality among almost all Jews today is that they have not gone the way of the early Messianic Jews and those affiliated with them over the centuries — the Christians.

A solid religious taxonomy classifies Jews and Christians — and Muslims — into one large family, sometimes called the Abrahamic faiths. But often the appearance and behavior of contemporary Jews, Christians and Muslims make it very hard to see the common roots and similarities.

There is a deep relationship among all believers in spite of their diverse religions. God is one, so all who seek him have much in common, no matter how strange they may seem, one to the other.

Taxonomy can be taxing.

Msgr. Archimandrite Robert L. Stern