Both counties have tiny Catholic communities and John Paul's 96th trip abroad is seen as a continuation of his ecumenical efforts to promote dialogue between leaders of different religions.
Bulgaria is the sixth Eastern Orthodox country to be visited by the pope, after trips to Romania, Greece, Georgia, Ukraine and Armenia.
The Orthodox churches of Bulgaria and Russia have a particularly close historical relationship, and observers are commenting that John Paul's visit is a further step toward melting the ice with Russian Patriarch Alexei II, who has so far adamantly opposed a papal visit to the world's most populous and important Orthodox nation.
"The Apostolic visit of the Holy Father is a historic event as far as it makes possible a meeting between Eastern and Western Europe," said the apostolic nuncio to Bulgaria, Monsignor Antonio Menini.
"This meeting will make everyone, here in Bulgaria as well, to rediscover the Christian roots on the European continent and the joint contribution that the West and the East can make to build a common European home," Menini said.
The centuries-long conflict between Catholic and Orthodox Christians has deep roots, and the process of drawing together the two churches is a difficult one, as evidenced by the refusal of the head of the Bulgarian Church, Patriarch Maxim, to attend the official welcoming ceremony in honor of the pope. Maxim even changed the timing of a Holy Mass in Bulgaria's largest Orthodox shrine -- in an effort to prevent John Paul from attending.
Nevertheless, the patriarch agreed, after much cajoling by the country's politicians, to receive the pope at his seat of power, the Holy Synod.
Since the fall of communism in 1989, Bulgaria's politicians have been eager to bring John Paul to Bulgaria, hoping that his visit would help overcome lingering suspicions that the country's secret service was involved in the failed attempt on the pope's life.
Several Bulgarian heads of state extended invitations, but the patriarch of the Orthodox Church never joined them. Finally a national committee to bring John Paul to Bulgaria was formed several years ago. It collected over 100,000 signatures, and their efforts were successful.
Authorities are worried that something untoward could happen to John Paul while in Bulgaria, and the security measures are unprecedented.
As a result, the center of the capital is virtually paralyzed. In order to get to work or to their homes in the restricted area, people have to have a special permit from the police.
The security measures extend to the pope's two scheduled trips outside Sofia and are bound to cause havoc in the daily lives of Bulgarians. Most of them accept the tight security and are welcoming the visit as a historic occasion, bound to improve the country's image abroad.
Polls conducted on the eve of the papal visit indicated that about 37 percent of Bulgarians responding to the surveys said they want to go the streets to welcome the pontiff. There are more than 8 million Bulgarians.
The culmination of John Paul's trip to Bulgaria will be his visit on Sunday to Plovdiv, the country's second-largest city, founded centuries ago and the center of Catholicism in Bulgaria.
There the pope is scheduled to preside over a 3-hour-long Holy Liturgy with Beatification, proclaiming as martyrs three Catholic priests executed by the communist regime in 1952.