The name "Pantheon" is from the Ancient Greek "Pantheion" meaning "of, relating to, or common to all the gods". Cassius Dio, a Roman senator who wrote in Greek, speculated that the name comes either from the statues of many gods placed around this building, or from the resemblance of the dome to the heavens. His uncertainty strongly suggests that "Pantheon" (or Pantheum) was merely a nickname, not the formal name of the building. In fact, the concept of a pantheon dedicated to all the gods is questionable. The only definite pantheon recorded earlier than Agrippa's was at Antioch in Syria, though it is only mentioned by a sixth-century source. Ziegler tried to collect evidence of pantheons, but his list consists of simple dedications "to all the gods" or "to the Twelve Gods", which are not necessarily true pantheons in the sense of a temple housing a cult that literally worships all the gods.
Given this paragraph about the Roman temple known as the Pantheon, tell me whether we definitively know why the temple was called the Pantheon.
We do not know why the temple was called the Pantheon. Even Cassius Dio, a Roman senator who would have had more context by nature of living and ruling in Roman society, was only able to speculate where the name came from.