Final answer:
Marcion was called 'the first-born of Satan' by Polycarp due to his controversial theological perspectives that led to him being excommunicated from early Christian communities.
Step-by-step explanation:
The individual called “the first-born of Satan” by Polycarp, an early Christian bishop, was Marcion. Marcion was a theologian who created a Christian sect that differed significantly from the proto-orthodox Christianity of the time. He believed that the God of the Old Testament was a lesser deity than the God depicted by Jesus. Marcion's beliefs were particularly controversial because they rejected the Old Testament entirely, and he was consequently excommunicated for his views.
Early Christianity was marked by significant theological debates and divisions, especially surrounding the interpretation of Jesus's teachings and the nature of the texts that should be considered authoritative. There was no single, unified set of beliefs or texts in early Christianity.