Marcionism is a big word that describes a group of people who believed in some different ideas about God and the Bible a really long time ago, like over 1,800 years ago.
Marcion, who was a man who lived in about 85-160 AD, was the leader of this group. He thought that there were two different gods, one was the anger god of the Old Testament, and the other was the loving God, Jesus' father.
Marcion believed that the grace and love of Jesus was not the same as the laws and angry God of the Old Testament. In this group, they also had a special edition of the Bible that had only specific parts that agreed with their beliefs.
However, many people who followed Christianity at the time did not agree with Marcion because they believed in faith in both the Old and New Testaments. Still, Marcionism was a popular belief system for a while before it faded out over time.