Crossing the Tiber is the euphemism to describe conversion to the Catholic faith. To get to the Eternal City, one has to cross the bridge across the Tiber.
Ten years ago, my wife and I crossed the Tiber spiritually, returning to the Catholic faith after nearly a lifetime of being evangelical Protestants. This week we have been blessed to physically cross the Tiber River to come to Saint Peter's to celebrate our tenth anniversary of coming home to the same faith passed from Jesus to his successors.
We prayed at Saint Peter's tomb and prayed at the site where he was crucified upside down. We saw the ancient obelisk that was the last thing he saw before he died. We received Jesus Christ in the Eucharist in the basilica of Saint Peter as countless saints and sinners have done since the construction of the Church in the early 4th century.
It is hard to describe the emotions we experienced as we realized we were now both physically and spiritually in the Church that was built upon the one who received the keys of the kingdom from our Lord and Savior, Jesus Christ. We are home by his grace and for his glory.