Nineveh, a murderous city of 120,000 people, converted after hearing the prophetic warning of Jonah (see Jon 4:11). Do you believe that there are cities in the world today which would repent en masse if someone would only speak God's word to them?
Saul was the worst sinner in history, a fierce persecutor of the Church (Acts 22:4; 26:10-11), and an accomplice in murdering the first martyr (1 Tm 1:13; Acts 8:1). Probably no one believed he would convert, become a great Christian missionary, and write much of the Bible. But he did!
Miraculous mass conversions all around us are just waiting to happen, if only a Jonah or Ananias (Acts 9:10ff) would obey God. A person, city, or nation may reject God 5,000 times. Yet who knows if the next time will be the time of conversion? We'll never know unless someone tries.
When we stand before the Lord on Judgment Day, we will not be judged on how many converts we made but on how many opportunities to share Jesus that we took or missed. The Church teaches: "The true apostle is on the lookout for occasions of announcing Christ by word, either to unbelievers...or to the faithful" (Catechism of the Catholic Church, 905).
"Listen to what I say: open your eyes and see! The fields are shining for harvest!" (Jn 4:35) Share your love. Share Jesus. You'll be surprised!