Sorry this took me a while, I was at work a lot this week and I didn’t really have the energy to write a reply after getting home!
Okay, so. You’re right that RE, Amun, Atum and Ptah all occupy the same role in the Egyptian “pantheon”, that of creator god. I’ll try to condense the complex nature of Egyptian religion into a reasonably brief answer. This answer exists in parts, and to discuss these there are a few things you have to keep in mind:
- There were different cosmogonological traditions in different areas/times of Egypt
- Religious beliefs shifted over time and Pharaonic history covers several millennia
- Syncretism is an aspect of Egyptian religion/mythology where two or more gods become associated with each other
Cosmogonies
In general we recognise four “main” (in terms of respective importance) creation stories, which we call cosmogonies.
The oldest and most well-known is the Heliopolitan tradition (Old Kingdom). In this cosmogony, Atum emerges from the waters of Nun and masturbates or spits to create Shu and Tefnut. Shu and Tefnut couple and give birth to Geb and Nut, who in turn birth Osiris, Isis, Seth and Nephthys. We call these nine gods the Ennead of Heliopolis.
Then we have the Memphite theology, which most likely dates back to the Ramesside Period (New Kingdom). Here the creator god is Ptah-Tatenen, who created the world according to his heart/will and the performative power of his words.
Third is the Hermopolitan/Theban tradition. In this case the divine primordial entity sprang from the Ogdoad - four couples of gods, each existing of a male personification of an aspect and his female counterpart. They are Nun and Naunet (primordial waters), Kek and Keket (darkness), Heh and Hehet (spatial infinity), and Amun and Amunet (that which is hidden).
Lastly there is the tradition of Elephantine, in which Khnum fashions the world and everything in it on his potter’s wheel, using silt from Nun.
There is some overlap between these cosmogonies. The Ogdoad of Hermopolis, for example, may have existed from the New Kingdom onward, possibly earlier, and thus coincided at least with the Memphite Theology. The Heliopolitan tradition is the oldest and “longest-running”, so to say, and thus overlaps with many of the other traditions.
Shift of religious beliefs
Part of the emergence of different cosmogonological traditions can be attributed to the passing of time. Over the centuries, Egyptian religious thought developed and changed. They did mostly attempt to fit their new ideas into the old mold, but sometimes a god who started out as relatively obscure found new importance.
Over the millennia of Pharaonic history, the chief god, for example, cycled from Neith, to Osiris, to Amun. This was a natural cycle and wouldn’t have affected the regular Egyptian populace all that much: in villages and settlements the people generally worshipped local gods, if they worshipped to any degree at all.
The sun god Re has been an important figure throughout Egyptian religious thought - or perhaps more specifically, “Re” was the most well-known name of the sun/creator god, since the creator god was usually anonymous.
That brings me to syncretism.
Syncretism
What you called “combined with”, we call “syncretism”. When one god syncretises with another, they merge into a divine entity that combines aspects of both while the original gods lose nothing of their individuality. So in essence, 1 + 1 = 3.
One of the most well-known examples is Re-Khepri-Atum. The obscurer solar gods Khepri and Atum became associated with/considered aspects of Re later in Pharaonic history. In this triad of sorts, Khepri was the morning sun, Re the midday sun, and Atum the evening sun.
Syncretism happened for a variety of reasons, for example because there was an overlap between aspects of the divine entities in question, e.g. isis-Hathor. Syncretising a more obscure god with a far more well-known one if they had an overlap of aspects was also a way priests could try to give their minor god more importance. Through syncretism we get gods such as Re-Atum, Amun-Re and Re-Harakhty, while we also still have the individual parts Re, Atum, Amun and Harakhty.
Now as for some more specific answers:
- Re, Amun and Atum are all creator gods as well as solar gods. They co-exist throughout most of Egyptian history, all in various and fluctuating levels of importance. In later periods Atum becomes the evening sun aspect of Re.
- Ptah is a creator god in his “own” cosmogony. He also plays a part as scribe to the (creator) god(s) in the Heliopolitan tradition, and in general is considered the patron god of writers/the writing art.
- Locality and passage of time both play a big part in the worship of certain gods. Chief gods and traditions could, and did shift.
I hope that clears up a lot for you! I can definitely recommend you pick up Gods and Men in Egypt by Françoise Dunand and Christiane Zivie-Coche for a far more in-depth discussion of the above information. It’s a very accessible book, both in terms of writing style and price!
TL;DR: Welcome to Egyptian religion, where everyone is also everyone else!
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