Here’s the real situation:
Only the line “he was brought up to heaven” (24.51) is omitted in some ancient witnesses. Among them are two Greek manuscripts (=MSS)—Codex Sinaiticus and Codex Cantabrigiensis, most of the early Latin witnesses, and one version of the Syriac tradition. However, all others have the words. The very earliest MS has this expression, as do the vast bulk of Greek witnesses—about 2000 in all for this portion of Luke. But it’s not the numbers that count. Instead, one has to weigh the evidence, not count MSS. The evidence for the omission is fairly strong; even the 1988 version of the NASB lacks those words. But it retains the words “he departed from them”—which may well imply the ascension.
One reason for the omission is that the ascension is repeated in Acts 1. Luke and Acts were originally circulated as two parts of one book. Luke intended for them to be read that way. The reason some suggest for the omission—that these scribes somehow thought that this was not true—is invalidated by Acts 1. There Luke tells us that Jesus was taken up from the disciples’ sight and was taken into heaven. Verse 9 is pretty much the same in all the MSS—including Sinaiticus and Cantabrigiensis. Verse 10, which says “and as they were gazing into heaven, while he was going, behold two men stood before them in white clothes...” has no variants at this juncture. Verse 11 says, “and they said, ‘Men of Galilee, why are you standing here, looking into heaven; this Jesus, who was taken up from you into heaven, will come in the same manner as you saw him leave into heaven.’” The only substantive textual variant is the second “into heaven”—omitted in Codex Cantabrigiensis.
The fact that that MS does not tamper with the first or third mention tells us emphatically that the scribe did not regard the ascension as a myth; rather, he omitted the second reference for literary purposes. Since the same scribe omitted that line in Luke 24.51, the reason must surely be his literary sensibilities rather than theological alteration. The ascension of Christ is thus clearly taught in all the ancient witnesses. It is never in doubt by any.
Finally, regarding using this issue to deny that the Bible is the Word of God: That is a naive argument. The reason is simple: What is at stake in these viable textual variants is NEVER any fundamental doctrine of the Christian faith. The death and resurrection of the God-man, Jesus Christ, and the necessity of faith in him for salvation, are all clearly taught. What is at stake in the textual variants is our particular understanding of any given text, not our embracing of the essentials of the Christian faith.