Princess Olga of Kiev (ca. 890–969) was not the nicest person you’ve ever met, at least before her baptism in either her 50s or 60s, but she may well have been one of the cleverest. She was the widow of Igor of Kiev, who wasn’t a Christian, but (according to some sources) would have liked to be—he was torn by a conflict of desire and realpolitik. He was also torn apart by the Drevlians when he tried to collect tribute from them twice in the same month. Olga thus became regent for their son Svyatoslav, who was three at the time.
The first thing to do, of course, was avenge her husband’s death, which she did in grand style. (Big sins make for big repentance, and also good stories.) First the Drevlians sent an emissary of twenty to convince her to marry their prince, who went by Mal (“Mal means bad in Latin”—obligatory gratuitous sci-fi reference). She had them buried alive. She sent word by return post that she had accepted Mal’s offer but needed a higher caliber of escort to escort her to Drevlia. Mal sent his best and wisest, and Olga ushered them into the bathhouse, locked the door, and torched the place. Next she invited the whole town to come to Igor’s funeral, and when they had enjoyed themselves at the hostless bar, she had 5,000 of them put to the sword. The remaining townspeople cried for mercy, promising to send furs or honey or anything else they had lying around if she would show them, well, mercy. She asked merely for three pigeons and three sparrows from each house. Delighted with this unonerous request, they agreed. Olga’s troops attached a string with a bit of sulfur to each bird, and released them to fly home. The whole town caught fire at once, and was completely destroyed. Those fleeing were captured by her army—some of them were killed, some sold as slaves, and the rest made to pay tribute.
She did other unpleasant stuff, but that’s probably enough for a picture of her needs-to-be-repented-of past (and cleverness). Some of our sources mark this as a great victory of the power of Christianity over the evil pagans. There, I’ve informed you of the existence of that opinion.
Whilst in Constantinople in either 945 or 957, after an unsuccessful military or successful business venture, and either before or after she turned over the rule to her son, she attended the divine liturgy at the Church of the Holy Wisdom (“Hagia Sophia” as it’s called by Greeks and by people like me who like to sound vaguely erudite). She was soon baptized, taking the name Helen, either after the reigning empress Helena Lekapena, or after Constantine’s mum, discoverer of the True Cross™ (Mar 6). Or both. At any rate she wasn’t named after Paris’ paramour, and that’s the main thing.
Returning home she set about evangelizing the people (we’re not told how well that went down with the (remaining) Drevilians), building churches (including the great Church of the Holy Wisdom in Kiev), and so on. Unfortunately the one person she couldn’t convert was her son, Svyatislav, who remained obstinately pagan to the end of his days. Olga reigned in Svati’s stead when he was out fighting wars, and might have tried something then, but mass baptism was on his no-no list, and that was that.
One thing she is famous for is bringing a piece of the True Cross™ to Kiev, one large enough to have “The Holy Cross for the Regeneration of the Russian Land, Received by Noble Princess Olga” written on it in (one hopes) macroscopic letters. After living (at least part of) her life in a Christian manner, she insisted that she be buried in a Christian manner, and she was. She is the patron of widows, converts, and the Byzantine Catholic Archeparchy (yes that’s a real word) of Winnipeg.