After the rapture of the first century church in AD 66, there were no Christians left on earth. However, the gospel had been preached to multitudes of people throughout the know world (Roman Empire) during the transition period, which would have included many who had either heard the gospel, or read some of the Bible scrolls, but who did not believe it and come to repentant faith before Christ’s Parousia in AD 66-70. They were not yet born-again to be included in the rapture of the church in AD 66.
After the rapture of the church in AD 66, certainly at least some of those unbelievers would have become Christians, especially as they lived through AD 66-70 and would have seen firsthand those prophetic events from Revelation coming to life right around them. Surely, the Holy Spirit would have used some of these living events to strike deep into the heart and bring alive some of the passages and stories they would have heard from a Christian earlier in their life. The “seed” had been previously sown for the Kingdom to continue to grow (Isa. 9:7). The original sowers did not need to be present any longer.
The earthly realm of the eternal New Covenant spiritual Kingdom (Church) began to grow after AD 70, slowly at first (as depicted in Ezek. 47:1-12). Some of the new Christians would have included these unbelievers who had previously had a “seed” planted within them. Many would have probably heard the gospel from their deceased or raptured parents or friends, while others would have come to faith after finding the many New Testament Bible scrolls which would have been scattered throughout the Roman Empire. These new believers would have been born-again in the very same way anyone comes to faith, and then been indwelt by the Holy Spirit, who guided them in truth as they grew as babes in the faith. They had the completed Canon of the New Testament to instruct them in doctrine.
The Word of God (Bible) is adequate to proclaim the gospel. The Word of God is a living book and its words have the supernatural power of God the Holy Spirit to convict the heart of sin and its need for repentance:
“For the word of God is LIVING AND POWERFUL, and sharper than any two-edged sword, piercing even to the division of soul and spirit, and of joints and marrow, and is a discerner of the thoughts and intents of the heart.” (Heb. 4:12).
“but these are written that you MAY BELIEVE that Jesus is the Christ, the Son of God, and THAT BELIEVING you MAY HAVE LIFE in His name.” (John 20:31).
“and that from childhood you have known the Holy Scriptures, which are ABLE to make you wise FOR SALVATION through faith which is in Christ Jesus.” (2 Tim. 3:15).
““Most assuredly, I say to you, he who HEARS My word and BELIEVES in Him who sent Me HAS EVERLASTING LIFE, and shall NOT come into judgment, but has passed from death into life.” (John 5:24).
“So then faith comes by HEARING, and hearing by the WORD OF GOD.” (Rom. 10:17).
All generations to come after AD 70 could still hear the gospel and believe in Christ through the written word.
“20I do not pray for these (apostles) alone, but also for those WHO WILL BELIEVE in Me THROUGH THEIR WORD; 21that they ALL MAY BE ONE, as You, Father, are in Me, and I in You; that they also may be one in US, THAT THE WORLD MAY BELIEVE that You sent Me.” (John 17:20-21).
Even though the apostles and all Christians had been raptured, these previously “seed planted” unbelievers had heard (or read) the gospel, and now believed, repented and were born-again after AD 70. They were the first offspring of the New Covenant marriage of Christ and His Bride in heaven (Rev. 19:7-9). The first trickle of the River of Life “emanating from the throne in the heavenly realm in Revelation 2:1-2” which Ezekiel sees “entering the earthly realm” coming out from under the door to the temple in Ezekiel 47:1. This trickle, which over time would continue to grow in width and depth throughout all history, and everywhere the river (gospel) would goes it will bring “medicine/healing” (save souls: Ezek. 47:12; Rev. 22:2). Christ has proven that His Words will never pass away, and His Kingdom never cease from growing! (Matt. 24:35; Isa. 9:7; Eph. 3:21).
See also related “Topic Studies & Terms”:
“The Perfect (That Which is Perfect) (1 Corinthians 13:9-12)”
For a more in-depth study see the related full “Study Series”:
“Study Series 16 Lesson 1 Rev. Dating, Intro Chapters 1 thru 4”