Start a new game with create your own ruler in 867 in Holmgardr. Make sure you turn Ironman on. Make a ruler with education- master philosopher, personality- just, gregarious and honest (these match the Slovianski Pravda faith virtuous traits), and other traits- theologian. Put any other spare points below 400 into Learning. Start the game, choose theology focus in Learning, run the game at speed 5 for about 18 months. As soon as you hit 250 Piety, click the religion button in the bottom left of the screen, click other faiths, click off only Nordic faiths, and convert to Slovianska Pravda.