[citation needed] In 1618, Smotrytsky returned to Vilnius where at the Holy Spirit Monastery he took vows as a monk and assumed the name Miletius.

This explanation, first advanced by Josyf Veliamyn Rutsky, appears in the papal Bull of Pope Pius IX canonizing Kuncewicz.

Orthodox Christians, in turn, attributed his conversion to worldly reasons: alleging that he was tempted by the money and prestige of an imminent appointment as Abbot of a Uniate monastery in Dermaniu.

He said that since 1615 he had waged an internal battle, growing stronger in the course of writing his texts; in them he defended the Orthodox dogma with diminishing conviction, suggesting that, even before chirotonią, the bishop did not agree with the spirit of brotherhood following his anti-Uniate publication from Vilnius.

This decision was the result of a deep spiritual crisis and the ultimate belief that the only thing that could help the revival of the Ruthenian people and culture was binding ancient Rus' lands with the Polish–Lithuanian state, and union of the churches.

Mironowicz claims that Smotrytsky's disappointing experience with a trip to two of the ancient patriarchates meant that he decided to devote all his strength to the union "of Rus" in one Uniate Church.

On July 6, 1627, Meletius Smotrytsky sent a letter to Pope Urban VIII, asking for forgiveness of his sins and acceptance into the Catholic Church.

While retaining his popularity among the priests and faithful in Belarus, he lost trust among both the Vilnius fraternity and Kyiv monks in monasteries.

On April 8, 1628, Cardinal Ludovisi in a letter to Alexander Zasławski demanded that the minister make a public profession of the Catholic faith.

The next meeting of the hierarchy took place on the sixth Sunday of Lent in Horodku, on property belonging to the Kyiv Pechersk Lavra.

In addition to Meletius Smotrytsky, it was attended by the Metropolitan Job, Bishop Isaac of Lutsk, Chelm Paisjusz and Peter Mogila, who was already igumen in the Kyiv Pechersk Lavra.

Hierarchs agreed thereby that contrary to earlier findings, Meletius Smotrycki published no new treaty discussing the differences between the rival churches.

The priest wrote that he had been in the council visited by representatives of the Cossacks, who threatened him with death if the Orthodox hierarchy agreed to join the union.

Also, the Catholic Church immediately after the council of Kyiv decided Smotrytsky was not fully devoted to the union, especially since it was still not publicly acknowledged that he had left Orthodoxy.

In September 1629 Smotrytsky took part in the Orthodox-Uniate Synod in Lviv, where he had already performed at the opening of the Uniate and Orthodox negotiations to convert to Catholicism.

The reason for such a decision was that the pope could have a negative assessment of Smotrytsky's attitudes at council in Kiev, where he pleaded not guilty to his conversion.

On February 16, 1630 Smotrytsky addressed to Pope Urban VIII, another letter in which he presented a new plan for the evolution of the Commonwealth, in which he suggested the use of force to spread the union by uniting the efforts of the Church, the State and the Catholic magnates.

Smotrytsky's ever-increasing intolerance resulted from his conviction that only by adopt the union Ruthenians would become a political nation, respected on par with Poles and Lithuanians and preserving their autonomy.

According to his account he was dying to ask you to insert his hand in the coffin of Pope Urban VIII letter informing him of the receipt for the Archbishop of Hierapolis.

The matter was discussed again in July 1635, and in October of the same year, a representative of the Congregation speaking to Rutsky to ask for a copy of the description posthumous miracle.

While the ultimate message of the Congregation stated that Smotrycki finally "rejected" letter of Patriarch theophany, doubts in this regard could be one of the reasons why the canonization process Smotryckiego was suspended.

Smotrytsky's work for Slavic grammar can be compared by its impact with that of Dionysius Thrax for Greek and that of Aelius Donatus and Priscian for Latin.