It seems that multitudes became Christians in the first century.
(Acts 2:41) 41 So those who gladly accepted his word were baptized, and on that day about 3,000 people were added.
(Acts 2:47) 47 praising God and finding favor with all the people. At the same time Jehovah continued to add to them daily those being saved.
And Acts 4:4 says, “many of those who had listened to the speech believed, and the number of the men became about 5,000”.
(Acts 5:14) 14 More than that, believers in the Lord kept on being added, great numbers both of men and of women.
(Acts 6:7) 7 Consequently, the word of God continued to spread, and the number of the disciples kept multiplying very much in Jerusalem; and a large crowd of priests began to be obedient to the faith.
Acts 9:42 This (healing) became known throughout all Jopʹpa, and many became believers in the Lord
(Acts 11:21) Furthermore, the hand of Jehovah was with them, and a great number became believers and turned to the Lord.
(Acts 11:24,26) 24 for he was a good man and full of holy spirit and faith. And a considerable crowd was added to the Lord. 26 So for a whole year they assembled with them in the congregation and taught quite a crowd, and it was first in Antioch that the disciples were by divine providence called Christians
Acts 13:48 When those of the nations heard this, they began to rejoice and to glorify the word of Jehovah,* and all those who were rightly disposed for everlasting life became believers.
Acts 14:1 Now in I·coʹni·um they entered together into the synagogue of the Jews and spoke in such a manner that a great multitude of both Jews and Greeks became believers.
Acts 18:8-10 And many of the Corinthians who heard began to believe and be baptized.9 Moreover, the Lord said to Paul in a vision by night: “Do not be afraid, but keep on speaking and do not keep silent, 10 for I am with you+ and no man will assault you to harm you; for I have many people in this city.”
David Chilton, in his excellent commentary on Revelation entitled, "Days of Vengeance" page 206 says
“The number 144,000 is obviously symbolic: twelve (the number of Israel) squared, then multiplied by 1000 (ten and its multiples symbolizing many; cf. Deut. 1:11; Ps.50:10; 68:17; 84:10; 90:4). St. John pictures for us the ideal Israel, Israel as it was meant to be, in all its perfection, symmetry, and completeness."
There is biblical support that the 144,000 and the Great Multitude are identical, that they really are the same group being described from two different perspectives, divine and human. John heard the (perfect) number; then he saw the actual group. This "hearing" then "seeing" sequencing is a literary device that THE APOSTLE John uses frequently in Revelation to display his images from different viewpoints. David Chilton demonstrates this pattern of hearing and seeing on page 213 of:”Days of Vengeance:”
“For example, in 1:10-13, St. John hears a Voice, then turns
to see the Lord; in 5:5-6, he hears the Lion of Judah, then
sees the Lamb; in 6:1-8, he hears a living creature say "Come!"
- then sees the object of the creatures command. The same
pattern appears in this chapter: St. John tells us, I heard the
number of those who were sealed (v.4); then, after these things
- after hearing the number of the redeemed - I looked, and behold
a Great Multitude (v.9).
There is one church indivisible; the Great crowd is not a piddly 8.5 million which we can all number; and the number of those sealed by the Spirit of God is a perfect number, representing a perfect innumerable body of believers. As promised to Abraham, Isaac and Jacob.