It's important to note that the Grand Inquisitor is not simply a made-up poem within the novel. He is a character who appears in a dream experienced by one of the protagonists, Alyosha Karamazov.
In the dream, the Grand Inquisitor, a powerful figure in the Roman Catholic Church, confronts Jesus Christ after his return to Earth. The Inquisitor argues that humanity is too weak to handle true freedom and needs the Church to guide them, even if it means restricting their freedom.
The story of the Grand Inquisitor is a powerful philosophical dialogue about faith, freedom, and the role of the Church in society. It's not intended to be a literal historical account but rather a thought-provoking exploration of complex ideas.