According to the dogmas of the Christian church itself, the first people to adopt Christianity were Jews. As a religion, it originated in the I century on the territory of the modern state of Israel, which at that time was one of the provinces of the Western Roman Empire. By the 4th century, Christianity spread throughout the empire, encompassing most of modern Western Europe, the Mediterranean and penetrated into Transcaucasia. If we keep in mind who first adopted Christianity at the level of the state religion, then this was Armenia, where it happened under Tsar Trdat III back in 301. As a comparison, in the Roman Empire Christianity received the status of state religion only in 382. The most important historical role in the adoption and spread of Christianity in Armenia was played by Gregory the Illuminator, who later became the high priests of this church - the Catholicos. In honor of him, unofficially, the Armenian Apostolic Church is also referred to in some sources as Gregorian.
If the question is about who first adopted Christianity in Russian lands, then historians have not reached a single opinion so far. In the study of this issue, it should be assumed that among the Germanic and Slavic peoples this religion began to spread especially actively since the liquidation of the Roman Empire. Then, by the 13th – 14th centuries, it penetrates the Finns and the Baltics.
The sources available today give very conflicting information about who first adopted Christianity, referring to the personalities of the historical figure. This is a very important clarification, since it should be understood that such transformations in society, such as the adoption of religion, especially at the state level, could not have happened without the organizing role of the state.
Traditionally, the date of adoption of Christianity in Russian lands is considered to be 988, when Vladimir was the Kiev prince. Some researchers call another date - 990 year. The problem is further complicated by the fact that the question itself contains some incorrectness. What is meant by the given: "who first adopted Christianity"? A historical figure, a people or a group of peoples compactly living in a certain territory ...
In order to remove from consideration the last two theses, we can confidently say that the process of Christianization of the lands of the Russian state was a long and very complex, sometimes reversible, process. Therefore, it must be investigated in relation to the history of each nation or nationality that lived in the structure of Ancient Russia.
Actual, in the question of who first adopted Christianity in Russia, of course, the problem of personalities remains.
The most frequently cited "Tale of Bygone Years" states that in 6496 (and this is exactly the year 988 according to the new chronology), "... Jesus Christ, who loved her, enlightened us," and Vladimir Svyatoslavich was the Kiev prince at that time. The adoption of Christianity by the classics of Russian history is also interpreted differently. They even used various terms to denote this event, obviously, each in its own way understanding it. So, V.N. Tatishchev in his works speaks of "the baptism of the Slavs and Rus", and N.M. Karamzin - about the "baptism of Russia." Therefore, a reducible approach to the answer can only be the approach associated with the search for a reliable historical figure, with the active participation of which this unique event occurred.
It is widely believed that baptism should be associated with the fact of the adoption of Christianity in Constantinople by the grandmother of Prince Vladimir back in 969 (or 957). The reason is the mention of the presence in the retinue of Olga a certain Gregory, according to guesses, the Christian confessor of the princess.
Even earlier dates and other personalities are called in connection with the study of the very history of Russian statehood. Thus, the Western European and Baltic chronicles say that Christianity was adopted after unsuccessful trips to Constantinople by the princes Askold and Dir, and this happened more than 100 years before Vladimir - between 842 and 867 years. In addition, in the lists of registers of Tsarigrad bishops of the late 9th century there are already records of the Russian diocese, which is listed under 60 - 61 numbers, and these registers, as you know, were filled in a strictly chronological order.
Thus, to date, the question posed in the title of the article has an answer only of a hypothetical nature.