Pope Constantine became Pope on March 25, 708 C.E. Do not confuse Pope Constantine with the Roman Emperor Constantine the Great (ca. 272-337).
Odo was never the pope.
Saladin was never a pope.
Saladin was not a pope.
A priest could only become pope if he is elected by the cardinals in conclave.
To become a pope, one must be at least 18 years old.
You become a pope when you are elected as such in a secret conclave held in the Sistine Chapel of the Vatican to replace a pope who as either died or retired.
There has never been a Pope John Benedict.
He was elected as pope on April 22, 1073.
Vatican City
The pope has always been a Christian.
Pope John XXII became pope on August 7, 1316.
Since Pope Francis is the first and only pope by that name, there is no need for a number after his name. If a pope in the future also decides to take the name Francis, he will become Pope Francis II and then the original Pope Francis will become Pope Francis I.