as powerful as the church was i dont think they had any armies to attck a nation or kingdom.. all they have is a voice and hopefully kings and royals would obey.
didnt one of the english kings scuff the church and started his own church in order to divorce and remarry his wive?
anyways if i were king i wouldnt be worrying about the church and the pope too much but i would be worrying about my kingdom and its ppl trying to teach and educate them with making them literate and improve their lives with my knowledge of agriculture and basic science like making soap and gun powder and how to make the ingredients with what nature provides.
even making mechanical inventions.. things like plumbing might be very crude and basic but would get the ball rolling to the point that they could become a modern society like ours in less then a few hundred yrs.. maybe in a hundred to 2oo yrs..
give them the will to want to improve... show them how fertilizer works which they should already know or alternating different plants for different seasons and how to replenish the soil which again should already know.
taking regular bathes and how fleas can be bad.. build basic telescopes to expand their understanding of the world.
teach them the scientific method.
even show them how to make very basic guns and bullets and principle behind it.. have competitions and rewards for the most ingenious firing mechanism for the rifles and guns..
i teach them how to make steel by increasing the carbon in the iron..
that is if i should choice to go back in time... i also open a church and reteach them about god and not leave out things in the bible but try to sway them from violence and focus more on compassion and understanding the principals of jesus... all while helping them learn to read..
maybe even build a printing press to mass produce media and cheaply.. the more you know the better chance of success make knowledge available to your kingdom rich and poor.
i would try to teach them that each person responsibility is to help one another and this includes even more so to the king and queens.