The Pope is the Vicar of Christ, in a continuous line stretching back to St. Peter who was commissioned by Him to lead His Church on Earth.
John Calvin was not.
Every pope was and is likewise just a fallible man.
They differ in that Calvin did not presume to blaspheme by calling himself "another Christ" or the "head of the church on earth."
Whereas, every pope does.
"But I say unto you, That every idle word that men shall speak, they shall give account thereof in the day of judgment." -- Matthew 12:36