Well before its founding, the area that would become the Kievan Rus was inhabited by several pagan tribes. The Viking you were told about was the Varangian warrior Rurik. Now the earliest primary source regarding Ruriks life is a compilation called the Russian Primary Chronicle, which was compiled by monks around the 11th and 12th centuries. Now according the Chronicle Rurik was a Varangian warrior who founded the city of Novgorod and ruled it until his death in 879 AD. Now, this Chronicle having been written some 2 odd centuries after the death of Rurik leaves something to be desired so whether or not he actually existed is still contested, as I stated above, primary sources regarding the subject are few and far between.

Now as to the religion. I cant say much for the Pagan religion practiced by the Slavs prior to their conversion to Christianity, but the Kievan Rus converted to Orthodox Christianity in the 988 when the Prince (now Saint) Vladimir chose to be baptized and become a christian. He also it would seem considered converting to Islam but opted out because of the ban on alcohol, supposedly stating "drink is the joy of the Russian".

Sources: A Concise History of Russia by Paul Bushckovitch is my go to for primers on Russian history.

Hope this helped answer your question!

It is a bit more complicated as the word Rus' changed its meaning over centuries. There's been much debate as to what the word stood for, with proponents of various theories roughly falling into two opposing camps. The so-called Normanists would posit the Rus' were Varangians/Vikings and the anti-Normanists would say they were autochtonous Slavs. The former position was formulated by German historians in Russia in the 18th century and it was opposed by what few historians of Russian origin there were back then. The anti-Normanist theory got a major boost in the Stalin era when it was considered unacceptable that the original 'Russian' state should have been founded by outsiders - and, worse still, by, well, uhm, basically Westerners.

The consensus now seems to be somewhere in the middle. Initially the Rus' were Varangians who settled around what is now called Novgorod, in the lands inhabited by an outlying Slavic tribe called the Ilmen Slavs. This tribe is associated with the Mound archeological culture that is attested in the area in the 8th to 10th centuries.

The first references to the Rus' are in Western sources and identify them as being of Norse origin. The Frankish Annales Bertiniani mention some envoys who called themselves Rhos coming into Frankish lands from Constantinople in 839. They claimed to be of Swedish origin.

The earliest Norse settlement in Novgorod lands would be Aldeigja, now known as Staraya Ladoga. It shows signs of Norse occupation since the 8th century (the oldest wood used in construction was dated to before 753). The most prominent settlement was Helmgard that later gave rise to Novgorod (literally, New Town in Slavic). This settlement pattern is well attested in Rus' lands - first there's an isolated Norse settlement in a defensible position by the water and some time later it is abandoned or declines in importance, but a few kilometres from that place a new and bigger trade town emerges that shows signs of joint Norse and Slavic occupation. Which is why Novgorod in its present location seems to date back to the 930s but Helmgard existed before that. The prevalent theory is Helmgard was the archeological site known as Ryurikovo Gorodishche, 2 km South of the city proper. It was occupied as a permanent settlement since the 9th century. (Here's a [paper] (http://www.sarks.fi/fa/PDF/FA4_73.pdf) on the site by Nosov who has been leading excavations there for decades).

The probably legendary founder of the Rus' state that the Old Rus' sources call Ryurik may have been Danish Viking chieftain Rorik of Dorestad. This Ryurik guy is said to have ruled in (Staraya) Ladoga from 862 and later moved his residence to Helmgard/Novgorod where he died in 879. Later his kinsmen allegedly moved the princely court to Kiev and expanded their power over lands in what is now Ukraine and Russia.

Coming back to your question, initially the word probably applied to the Norse elite in these lands. But later the Norse and Slavs seem to have coalesced into a single group, particularly as they were expanding into ethnic Finnic and Permian lands in the North East along the Upper Volga and its tributaries. The Norse and Slavs seem to have expanded together along what later came to be known as the Volga trade route that ultimately led to the Caspian and Persia, the source of the silver Arabic dirham that was the preferred currency in the early Rus' state. Both groups were newcomers among a foreign and probably hostile population which helped bring them together. Their main settlement in the area was Rostov, attested among the main Rus' urban centers in 862. (Once again, the modern town of Rostov is somewhat younger, the initial settlement being the so-called Sarskoye Gorodishche). Well, by the early 10th century, when Rus' warriors from Rostov were joining Kiev's attacks on Constantinople they were led by a chieftain with a clearly Slavic name, Potanya. By that time the Rus' probably included both Varangians and Slavs.

Much of what was written about the Rus' (that is, the Varangians/Vikings that are said to have settled near what is now Novgorod and imposed their rule on the Slavs, whom the term 'Kievan Rus' is derived from) is highly mythologized, and it's difficult to ascertain exactly what the relationship between Varangian settlers in what would become the Kievan State and their Slavic subjects was. Traditional Russian history would have it that the Slavs voluntarily invited the Rus' to settle in their lands and rule them because they were incapable of governing themselves or protecting themselves from invaders. You can understand why it's best to take this with a grain of salt, partly because the primary source that this anecdote comes from, the Primary Chronicle, was written centuries after the events described, and it was used by the Grand Princes to legitimize their rule.

The Varangians came in the 9th century. After they arrived, the majority of the people in Rus' were still Slavs (as well as Mordvins and some other groups) , but the Varangians became a ruling class over them, establishing a state society. Over time, the Varangians intermarried with Slavs, and eventually became indistinguishable as an ethnic group. All the Tsars of Russia claimed descent from the Varangian settlers, until that bloodline died out when Ivan IV (AKA Ivan the Terrible) killed his intended heir and his other son died childless, after which the Romanov dynasty came to power following a very chaotic and bloody period of unrest.

We know precious little about the pagan traditions of the Slavs prior to the adoption of Christianity. There were numerous deities that varied in importance depending on where you were and who was worshiping. Vladimir, the Grand Prince of Kiev, converted to Orthodox Christianity in 988 both to create an alliance with the Byzantine Empire and to have a single religion to unite the disparate beliefs and ethnic groups that existed within Kievan Rus'. The conversion was involuntary, and upon his return to Kiev he forced all of his subjects to go into the river to be baptized, but what ended up happening was the new Christians combined many of their old pagan traditions with the new theology, resulting in a highly syncretic Christianity, something that occurred elsewhere in Europe as well.

If you want an accessible, general overview of Russian history that also goes into detail in important topics, then I highly reccomend Russia and the Russians, by Geoffrey Hosking. People will frequently recommend Russia Under the Old Regime by Richard Pipes as well, but it's quite outdated at this point, though definitely still worth a read if you want something a bit shorter.