Always keep in mind that all men are born in the same state, and nothing elevates more but the humility, nothing sinks more than pride and malice.

King Saint Stephen dedicated the Admonitions to his son Saint Emeric, a short set if instructions, a so called mirror for princes, a guide on how to rule, about the ten pillars of kings. The actual author seems to be a high priest, close to the king, but it is surely was ordered by Stephen, and contains thoughts that implies he also gave his opinions what to cover and how.
It's fun that the introduction notes: soft things make you soft. Emeric as a child could not yet face the hardships of military campaigns and battles, which were important themes in the life of the adult king Stephen, and he needs not just the nourishment of soft mush he was fed until now, but has to taste the sour wine of life.

I. Keeping he Catholic faith
For starters this is an interesting point. He says only those can gain the title of the king who are faithful and Catholic. We shouldn't forget that when this was written, and when the Hungarian Kingdom formed, when the Hungarian rulers gained the kingship, the title Rex - instead of Dux, elevated from there - the conflict between Rome and Constantinople was deeper and deeper and culminated in the 1054's Great Schism. So firstly this point shows only Christians can become kings, it is a Christian title. Second while at that point still everyone was Catholic, perhaps this is a refusal of Orthodoxy.
The work mentions the importance the belief in the Holy Trinity as is, and warn against those who want to change or reduce or inflate it. Warns about heresy.
II. Honor the Church's order
The Church as institution not the building or ritual. As the title says, honor it, defend it, and make it grow.
III. Respect the high priest
The advice is: they should be the key officials. Absolution is in their hands and they can tie men to sin too. On the other hand if one of them turns to sin, the king should warn him and make him find him the path back to righteousness (or snitch on him to the Church).