It gets confusing :)
At first Christianity was spread by Rome ( all short skirts,gladiators and Et Tu Brutas, the Empire ), thats how it arrived amongst British Celts first but the Romans (still short skirts and gladiators,but a slightly different version of Empire) who spread christianity first in the East were eastern Romans and came from the Arian tradition ( Jesus as man not God).
Than you skip forward a couple of hundred years and the *official* church of Rome has been born and thats when you get Papal delegates going round Europe telling all the other christians that they were heretics and converting them to the new version of christianity,ie,Romanised....confused? You will be :)
As for specifically the Celts in what is now Poland,i'll sit back and learn from others as my speciality is western history in the early medieval/dark ages :)
(Poland became officially Christian in Roman way)
Yes, *they say* the same about Britain in the early 7th century even though there had been Christians here since the late first century and it was already the dominant religion of the Roamno celtic british for about 3 hundred years before the Gregorian missions :)
BTW, Im sure a Slav could also be a Celt and vice versa seeing as neither is a genetic ethnic identity but rather a cultural and linguistic identity.