Bogdan's son and indirect successor Lațcu of Moldavia (1365-1373) invited a delegation from Rome, promising his and the people's conversion to Catholicism and asked Pope Urban V to send missionaries and erect a Latin diocese in his principality's capital, Siret.
On 24 July 1370 the Pope instructed the archbishop of Prague and bishops of Bratislava and Kraków to verify/complete the sincerity of Laţcu (although his wife remained Orthodox) and mandated them to erect such diocese covering the Moldavian state.
Pope Gregory XI established it in 1371, exempt (i.e. directly subject to the Holy See); Polish Franciscan Andrzej Jastrzębiec was consecrated first Bishop by archbishop Florian Mokrski of Kraków.
The cathedral, dedicated to John the Baptist, was built by queen Margareth, Catholic kin of the Hungarian royal family, which in 1377 had invited Dominicans to Siret.
However prince Laţcu favoring of Catholicism met grave opposition from the Orthodox clergy, while effective Latin converts were concentrated in the north of Moldavia, near Catholic neighbour kingdoms of Poland and Hungary.