We pledge to sincerely follow the guidance of our High Priest, who has solely possessed the Law transmitted to the succesive High Priests since True Buddha Nicheren Daishonin.
Is the sentence correctly phrased? Is 'has (solely) possessed' the correct verb? Could the sentence be phrased in a better way?
I'm not a teacher, but I would suggest "who alone has possessed The Law..." or "the sole possessor of The Law"