Landed vassals will accept with opinion at 36+ (Bön and unreformed Romuva landed vassals require higher opinion, as do reformed religions with any of Unyielding, Dogmatic, Defenders of Dievas, or Survivor of Ukko).
You may ask your courtier or direct vassal to convert to your religion if: Note: when a character converts to another religion, their children below 6 will also usually be converted.