Paul speaks of some 500 brothers to whom the risen Christ appeared, most still being alive when he wrote that around 55 AD (1 Corinthians 15:6). Further, Paul adds that they were witnesses of the risen Christ before his own Damascus road conversion. This means that, before that day of Pentecost in A.D. 33 when the witness of the new church kicked off Big Time, there were over 500 Christians, at the very least. But all the believers who were daily in Jerusalem’s temple waiting on the promised outpouring of the Spirit at Pentecost numbered 120. (Acts 1:12-14, 2:1) Then, the very day the Spirit was poured out on them, Peter’s preaching resulted in about 3,000 converts that same day! (Acts 2:41) Here’s a list of Acts references of more converts:
2:47 – Daily conversions.
3:4 – Total now about 5,000 men but not including women and children. Perhaps 10,000 total?
5:14 – More and more believed and were converted.
5:42 – What converted people was the message, “Jesus is the Messiah”.
6:1 – The believers “rapidly multiplied”.
6:7 – The believers “greatly increase” in Jerusalem and MANY Jewish Priests converted too before Stephen was martyred. Then began great persecution and the believers were scattered.
8:4-7 – Philip in Samaria city – MANY converted.
8:12 – MANY men and women were baptised.
8:36 – The Ethiopian chancellor was converted, baptised and went to Ethiopia.
8:40 – Philip then preached from Azotus to Caesarea.
Ch. 9 – Saul converted – days later preached Christ in synagogues, could not be refuted (9:22).
9:31 – The Church had peace, growing stronger in numbers and in faith.
9:35 – Peter in Lydda & area – HUGE NUMBER of conversions.
9:42 – In Joppa, the WHOLE TOWN believed in Jesus.
10:47 – Cornelius and his household believed and were baptised.
11:21 & 24 – Peter again, MANY people believed and were baptised.
Ch 12 – Persecution in Jerusalem. 12:24 – “MANY new believers”.
13:12 – Governor Sergius Paulus of Paphos became a Christian.
13:43 – Antioch (Pisidia) MANY believers.
14:1 – Iconium – a GREAT NUMBER of Jews and Greek believers.
14:21 – Derbe – MANY disciples made.
16:5 – The churches grew larger EVERY DAY.
16:14 – Philippi – Lydia and her household converted.
16:31-34 – Philippi jailer and his household believed and baptised.
17:1-4 – Thessalonica – some Jews, MANY Greek men and prominent women converted.
17:10-12 – Berea – MANY Jews and MANY prominent Greek men and women converted.
17:22-34 – Athens – “Some”, including Dionysius and Damaris and others.
18:1-3 – Corinth – Priscilla and Acquilla, Jews.
18:7-8 – Crispus, a Jew; his household and MANY others.
18:9-11 – After 18 months ministry, MANY converts.
18:24 – Ephesus – Apollos’s preaching – 12 men converted (19:1-7).
19: 8-10 – Asia – 2 years ministry.
19:18-20 – MANY became believers due to powerful effect of the preaching. [20:7 met on Sunday]
21:20 – “How many THOUSANDS OF JEWS have also believed”.
28:17-31 – Paul in Rome with a church already established there plus some Jewish leaders believed, then 2 year’s ministry.
One year into Christianity, the believers had increased from around 1000 to 10,000. Perhaps by AD 60 there were 100,000.
Many of them were in the Roman army and so they took the Christian message with them all over the Empire. Then came persecutions of the Christians, which caused many of the survivors to scatter, to escape it. There were 40,000 martyrs that died in the year 95 A.D. alone, but that left very many more Christians who escaped. Halleys Bible Handbook (Henry Halley) Zondervan reprint 2012 heading: The Age to Come (page 843): “The second Imperial persecution was under Emperor Domitian (A.D. 95). It was short but extremely severe. More than 40,000 Christians were tortured and killed.” McClintock & Strong Biblical Cyclopedia (online) confirms this.See also “History of the Church” by Eusebius and his description of early persecution of Christians under Domitian. Then, in the province of Egypt, there were 144,000 Christian martyrs during one of ten bouts of persecution. Another 700,000 died as a result of fatigues encountered in banishment or under enforced public works.
Converts to Christianity in those times were never forced. It was only much later, after the Church got mired in politics that the sword was used to threaten people's lives if they didn't convert - utterly against everything Jesus said and stood for. The printing press was not used in Europe until the late 1400s, by which time there were many millions of Christians all over the world.