Christianity arrived here in the 9th Century (and a bit earlier by some accounts). and had a tumultuous history alternating between growth and oppression.
The communist era was a time of great persecution for Catholics, nominally because Catholicism was considered the religion of fascism. Bulgarian communists also deemed Catholicism a foreign influence. Priest were tried on trumped-up charges in 1951-52 and four of them were executed. Then all Church property in the country was confiscated and parishes, schools, colleges and clubs were closed.
The Communist regime fell in 1989, and religious freedom was restored and by 1990 relations with the Vatican were restored. Pope John Paul II paid a three-day visit to Bulgaria in 2002, visiting Sofia and in Plovdiv, which helped to cement those relations.