The Protestant Reformation turned society upside down in countries under the aegis of the Vatican, but did anybody pay any attention in Orthodox countries? If so, who did they side with?
The impact was mostly limited, despite the efforts of Patriarch Cyril Lucaris (died 1638) to bring the Orthodox church and the protestant churches together. There was also some proselytizing which went both ways at various times, but this also the case with the Catholic church. In areas under Ottoman control, the Orthodox church was understandably far more preoccupied in dealing with Islam.
Although there was common ground in that both the Orthodox church and Protestant Reformation rejected the supremacy of the Pope in Rome, the differences between them were otherwise too great for the large majority of the orthodox hierarchy who regarded some of the teachings of the reformation to be heretical.
In a 2017 paper given by Fr. Panayiotis Papageorgiou Ph.D, a researcher in Early Christian Studies, history and theology, the author starts out by stating:
...from the perspective of the Orthodox of that time as well as the modern Orthodox world, the Protestant Reformation is a purely Western Christian issue. Even today, most Orthodox still do not know much about it, both as to why it happened as well as the intricacies of the issues and the theological diversity which subsequently arose out of the movement.
This was not for lack of trying to create an understanding between the two on the part of Lutherans, but their overtures were ultimately rejected. In 1559, an approach was made to Patriarch Joasaph II but no reply was received, probably partly because
There is abundant evidence that the proselytizing activities of both Protestants as well as Roman Catholics in the Eastern European Principalities populated by Orthodox Christians worried the Patriarchate of Constantinople at that time.
For example, in Transylvania
Calvinist preachers first became active in Oradea in the early 1550s....The government also exerted pressure on the Romanians in order to change their faith. The Diet of 1566 decreed that a Romanian Calvinist bishop be their sole religious leader.
More importantly, though, the documents presented to the Patriarch by the Lutherans
". . . embarrassed Joasaph and the Holy Synod. A brief glance at the Confession of Augsburg showed that much of its doctrine was frankly heretical. But it would be undesirable to spoil relations with a potential friend. The Patriarch and his advisers took refuge in the favorite device of oriental diplomacy. They behaved as if they had never received the communication, which they carefully mislaid."
Source: Stephen Runciman, 'The Great Church in Captivity', cited in 'The Protestant Reformation and the Orthodox Christian East'
Later, Patriarch Jeremias II (1572–1595) was approached
by Lutheran theologians from Tubingen university who wished to gain his favorable opinion to assist them in their struggle with Counter-Reformation theologians over issues of justification theology.
Source: John Anthony McGuckin, 'The Encyclopedia of Eastern Orthodox Christianity'
However, the approach was again ignored at first and - when that delay ran its course - a polite but firm rejection of the proposal for a unification of the two churches. Several further attempts were made by the Lutherans over the next ten years until
Patriarch responded for the last time in the Summer of 1581. This time he recapitulated the points of disagreement and begged for the correspondence to cease: "Go your own way", he wrote, "and do not send us further letters on doctrine but only letters written for the sake of friendship."
...As patriarch, Lukaris actively worked to establish better relations with both the Calvinists and the Anglicans, even presenting King Charles I of England with an early Greek manuscript of the Bible. In 1629, influenced in large part by the theology of Antoine Leger, he affixed his name to a Confessio Fidei (which he also may have authored), an attempt to harmonize traditional Orthodox thought with Calvinist theology.
This confession proved deeply controversial, though, and was rejected at several synods. Further,
His enemies, including Cyril Kontaris (later Patriarch Cyril II) and the Jesuits, frequently conspired together against him, allegedly offering the sultan money in exchange for Lukaris’s life. In 1638 he was accused of inciting the Cossacks against the Turks and executed by agents of Sultan Murad IV.