Some people make the assertion that the stories in the Bible were stolen from other, older religions around at the time and the story of Mithras is the Deity that the early Christians borrowed most heavily from when making up their own Deity.
Just because Mithras was born of a virgin, in a cave, on December 25th, his birth attended by shepherds, had twelve companions, promised his followers immortality, performed miracles, sacrificed himself for world peace, was buried in a tomb and rose after three days and will return in the last days to raise the dead and judge them does not necessarily mean that his story was rewritten with the name 'Jesus' tippexed in.
The truth is that at that time there was plenty of other Gods around who were crucified and resurrected who the Christians could have plagiarised from, like the Sumerian goddess Inanna, or Ishtar, who was nailed up and subsequently resurrected or Greek Attis who was crucified on a tree, buried but on the third day the priests found the tomb empty.
The Indian Buddah died on a cross, buried but arose again after his tomb was opened or the other Indian Krishna who died on a cross but came back to put in an appearance three days later.
To say that the Biblical stories of Jesus's Easter weekend were based on Mithras is even more off when you hear that Osiris of Egyptian legend was betrayed, crucified between two thieves and then buried in a tomb from which he arose on the third day.
If you hear non-believers trying to refute the story of Jesus by saying it is stolen from Mithras then you can point out that actually, it was probably stolen from a lot of stories about people being crucified and resurrected around at the time.
Then hope they shove an Easter Egg in their mouth and don't have any follow up questions.