What does it mean to "swim the Tiber"?

[NVBurch]

I know what people mean when they say “swim the Tiber” (convert to the Catholic Church), but where did the expression come from? What does it literally refer to?

 

[Michelle_Arnold]

The Tiber is a famous river in Italy and, in mythology, is believed to have been the river into which the legendary twins, Romulas and Remus, were thrown. The children were said to have been found by a she-wolf who cared for them. Romulus, after killing Remus, went on to found Rome in 753 B.C. Those who have converted to the Catholic Church often say that they “swam the Tiber” to indicate metaphorically that they have entered Roman Catholicism.