The sources give the exact date (year) of the baptism of our first historical ruler, Mieszko I. Since we have no reason to doubt their accuracy, the jubilee celebrated in 1966 was not the result of a convention, but actually the jubilee of the millennium of the Christianisation of our state. It was undoubtedly the most important moment in our history. Without it, we would not be what we are, because if we had not embraced Christianity, we would not have survived as a separate, living and developing nation. If we had lost the state sooner or later, we would certainly have lost our national individuality. Having adopted Christianity from the Roman Church, we became part of the Latin cultural world. In the following article, the author analyses the available historical sources on this important event in the history of our nation.