Please tell us what they say then, and where they say it so that we can look it up ourselves, because this is not something that is widely accepted as being true, even among Protestants.
Irenaeus calls him the Father of All Heresies. All the great heretics from the Valentinians to the Marcionites to the Manicheans were followers of his and his heresies. His followers called themselves "Christians", so as to blend in with the real Christians in Rome and elsewhere and be indistinguishable.
The New Schaff Herzog Encyclopedia of Religious Knowledge, Funk & Wagnall, 1910 states: "The 25 year episcopate of Peter at Rome is evidently due to the statement of Justin Martyr regarding the labors of Simon Magus at Rome".
And the Hastings Dictionary of the Apostolic Church states that "the author or first representative of this baptized heathenism . . . is Simon Magus, who unquestionably adulterated Christianity with pagan ideas and practices with the aid and the sanction of Christianity [so-called] to set up a rival universal [catholic] religion".