Complex sentence: He is supposed to have been either a priest's son or a converted Jew, and was highly educated, relative to the times he lived in, knowing as he did the Russian and Polish languages and being somewhat of an expert in liturgical matters.
Split into simpler sentences: He is supposed to have been either a priest's son or a converted Jew, and was highly educated relative to the times he lived in. He spoke both the Russian and Polish languages and was something of an expert in liturgical matters.