Let us start with contemplating about three things:
-Nomadic cultures seemed to have favoured some kind of shamanic spirituality, with earth and sky as main entities and independent figures as facilitators.
-Peripheral organised settling cultures show structures of an animistic concept, with a cast of priests, taking care for local entities and the rituals in the cycle of the year.
-Centrally organised empires need a regulated, concentric cult with a main entity and dogma.
The journey is for the nomads a main part of their life, only logical the spiritual caretaker is a wanderer in between the worlds. Earth and Sky rule the everyday life, so do their powers on the other side. The Thunderstorm and the Wind generally are of great influence, also as the connecting element between earth and sky, so their blessing is always wanted. The believe, that the soul wanders on in the afterlife is common.
The settlers are much more interested in the well being of their crops and cattle, so the contact to the very earth they want to remain on is important for them. Also, that that very place is blessed with the right amount of sun and rain to the right time. These people don´t need anyone to go out and tell them where to find the big rendeer herds anymore, they need someone to call the rain to their field, someone holding an eye on the movement of the sun for time measurement. The believe, that the souls of the deceased remain “living” in their graves, is not unknown. Fully furnitured grave chambers go along with that believe.
To rule large amounts of people and land is the focus for a monotheistic cult, already early egyptian pharaohs demand solitary worship, worth mentioning also the emperor cult of Ancient Rome. The central cult also collects the donations and offerings centrally and is so holding the economical control. The spiritual or even physical disadvantage not following the dogma is of great importance to keep up mind control and homogeneous behaviour. Rome, ruling big parts of Europe with massive military force first managed to remain in power and expand global after all with the turn to Christianity.
As a connecting element, we have syncretism, usually appearing, when cultures undergo a change in cult and culture. It means that religions blend and exist parallel to each other without a regulating element.
In the viking age we see in Scandinavia very clear signs of christianity slowly sinking in in a spirituality of settlers, wich still bears marks of the old, nomadic roots of the early eurasian cultures. Like the nomad tribes of the Siberian Steppe 500 BC, the Vikings still bring great horse sacrifices with their dead nearly 1500 years later. At the same time they already embrace the new religion from the south. We know cast forms, where with one melting pot a Thors Hammer and a cross can be cast. Christ is mentioned among the older gods in the majority of full rune line poems we know today. A big number of rune inscriptions is of christian nature. Even the highest god, wide worshipped Odin, undergoes a change. The angry Wotan, raging in the sky as dark ruler of souls but living under the grave mounds moves up to heavenly realms while his name slightly changes northwards.
On many Shaman and Udugan coats we still to this very day find crosses, none of these healers would be shy to use blessed water, if it helps.
Traditional Shaman burials are also found containing crosses and other christian symbolism.
In the Americas some of the First Nation churches even practice consumption of trance inducing hallucinogens during Sunday service.
Since shamanism technically is not a religion, it mixes very well with all other spiritual lines. It is the oldest way of seeking answers to the unanswerable, always open and always curious, since we are all wanderers on the wide open plains by nature, where it is space for everyone. The nomadic hunter and gatherer period is the longest form of existence for mankind. Nothing else left deeper traces in us. The nomad, a permanent guest, has no reason to see a foreign cult as hostile. The settler, living on the mountain couldn’t care less about the maritime religion of his fishing cousins at the sea.
The evidence shows us that christianity in the very beginning in the north was tolerated and loosely embraced. Christianity at that time also had no problem to adapt to local habits and absorb some cults, turning them in their own.
The people we see as the old pagans today had space in their temples for the newly arriving Crucifix, just the other way around it turned out to be a little bit difficult.
A monotheistic minority can be enriching, a monotheistic majority historically seen strives to Absolutism.