"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".
Why don't you finish the sentence:
The twenty-five years' episcopate of Peter at Rome is evidently due to the statement of Justin Martyr regarding the labors of Simon Magus at Rome (see above), combined with the tradition of Peter's residence in the same city, especially as it would seem that the Roman Church had actually been formed early in the reign of Claudius through the indirect influence of the Petrine Christianity of Palestine.
Here is the applicable page from Scaff-Herzog:
You can read the whole article here.