Pope Hadrian (Adrian) died Christmas, 795. He ruled from February 772 to 795 & had wanted keep the papacy independent of the power struggles between the Eastern Empire and the western kingdoms.
On the right is a painting of Adrian looking to Charlemagne for succor. Artist is unknown, alas.
Adrian had also promulgated the veneration of icons, as well as the teaching of their proper handling and creation and supported the suppression of the iconoclasts; Charlemagne was so angered by this, he commanded Theodulf of Orléans to write his argument in the Libri Carolini opposing images, as well as refuting the Second Council of Nicaea, which is todaymodern day Iznik, Turkey, click here to view the area.
In response to Charlemagne’s Libri, Adrian replied by letter and anathematized (threatened excommunication) to all who refused to venerate the images of Jesus, the Virgin Mary, or any of the saints. The dispute remained unsettled throughout the rest of Adrian’s reign.
Notwithstanding this, an ecumenical synod was held at Frankfurt (or Frankish territory) in 794 which obviously agreed with the Franks and condemned the icon practice, stating that it had come to their attention that there were people who were “worshipping the creature and not the Creator”.
This Council, also endorsed an addition Creed of the of the filioque (and the Son) clause because that was how the Franks understood the divine triune. This too was lambasted by Pope Adrian as well as, surprisingly, his weak successor Pope Leo III who was an iconoclast.
This clause, referred to as the infamous Filioque, became a major issue between the Eastern and Western churches, with the former rejecting it wholesale and the latter accepting it, still. I believe the Protestant churches do as well except for the Calvinists and perhaps other minor sects. I have an article that John Wesley, founder of the Methodists, agreed with Calvin but I have not read it thoroughly to ensure that.
Finally Adrian rightly opposed the heresy Adoptionism, that had come out of Moorish Spain. (The Moors in Spain was the Moslem invasion into the peninsula). This idea was that Jesus was born normally, as a normal child of Joseph and Mary but was later adopted by God as his son as a result of his righteousness. This idea was bandied about for a long time, and is often used against Christians today.