When the times of paganism passed, when Christianity ceased to be persecuted and was recognized as a world religion, it would seem that all differences should be resolved. But sometimes only Ecumenical Councils could resolve emerging conflicts, refute heretical teachings - there was no peace even within the Church.
The first Ecumenical Council was the Council of Nicaea, convened in 325. The reason for this was the widespread teaching of the Alexandrian presbyter Arius. Its essence was to deny the identity between God the Father and God the Son. He argued that Jesus Christ was created by the Lord, but is not his incarnation. Such a thought fundamentally refuted all the dogmas of Christianity, and therefore initially the teaching of Arius was rejected by the Local Council. However, the proud presbyter refused to recognize the decision of the Council as legitimate and continued to win over the faithful.
Then Emperor Constantine invited bishops from all over the world to the Ecumenical Council in the small city of Nicaea (now called Iznik and located on the territory of modern Turkey). Some of the representatives of the Church present bore traces of torture on their bodies,received in the name of true Christianity. Bishops supporting Arius were also present.
The debate lasted more than two months. During this time there were many discussions, speeches by philosophers, clarifications of theological formulations. As the legend tells, the manifestation of a divine miracle put an end to the bickering. As a unity of three principles, he gave an example of a clay shard: water, fire, clay give a single whole. Likewise, the Holy Trinity is essentially one God. After his speech, fire appeared from the shard, water appeared and clay formed. After such a miracle, the Nicene Council finally rejected the false teaching of Arius, excommunicated him from the Church, approved the Creed and established 20 rules of church discipline, determined the date for the celebration of Easter.
But this Church Council has not put an end to this issue. The controversy continued for a very long time. Even now, their echoes are still heard - Arianism formed the basis of the teachings of Jehovah's Witnesses.
In addition to the Council of 325, there was also the Second Council of Nicaea, convened by Empress Irene of Constantinople in 787. Its purpose was to abolish the iconoclasm that existed at that time. In fact, the Empress made two attempts to convene an Ecumenical Council. But in 786, the guards supporting the iconoclasts burst into the Temple of the Holy Apostles in Constantinople, where the Council began work. The holy fathers had to disperse.
Having resorted to considerable tricks, disbanding the old guard, recruiting new troops, Irina nevertheless convened the Cathedral in787, but moved it from Constantinople to Nicaea. His work lasted a month, following its results, the veneration of icons was restored, they were allowed in churches.
However, this Council of Nicaea also failed to fully achieve its goal. Iconoclasm continued to exist. The iconoclast movement was finally defeated only in 843, at the Council of Constantinople.