Explanation:

After the death of Jesus and the destruction of Jerusalem, Christianity as we know it was beginning to take off amongst Greek and Roman Gentiles thanks to the efforts of people like Paul.

However, this created a problem amongst early Christians because a lot of them had simply never read the Torah or Old Testament. They were just convinced purely by the teachings of Jesus and had been heavily influenced by Platonism as a base philosophy. And when Jewish Christians from Jerusalem and Palestine met these converts, they tried to teach them about the "Old Testament".

Unfortunately, the Gentile Christians often refused to believe the god of Jesus and the god of the Jews was the same god because they thought the Old Testament god was wraithful and evil (my favorite example is when god let bears kill some kids for making fun of a bald guy 2 Kings:23-25).

One such group of Christians were the Marcionites founded by Marcion of Sinope. It claimed the god of the old testament was the Creator deity, the Demiurge. They aren't a gnostic sect because Marcion was using what would become canon such as the writings of Paul and a source of Jesus sayings. This is opposed to the Valentinians who used revealed or gnostic information to form their theology.

Unfortunately, we mostly know about these early so called "gnostic" sects of Christians through condemnations from early church fathers. We have a good portion of Marcions cannon texts, but not a complete picture of their church. But they were significant at the time. Early Catholics had official instructions for their congregation to ask for a "Catholic Christian Church" when they travelled because a Gnostic church was just as likely to be in the town as a Catholic one.

they right tho