Al Case (AKA RPCman), a former Latter-day Saint, under the section, "Influenced by the KJV of the Bible," raises the following objections to the Book of Mormon:
Why don't the Book of Mormon quotes from out of the Old Testament agree to earlier Latin, Syriac, Coptic, or Patristic texts?
Example: Matthew 5:27 and 3 Nephi 12:27 "by them of old time" not included in earliest Greek (should have said "to them of old")
Matthew 6:4, 6, 18 and 3 Nephi :4, 6, 18 "openly" added later
Matt 6:13 and 3 Nephi 13:13 "lead us not into temptation, but deliver us from evil" should have said, "and do not bring us to the time of trial, but rescue us from the evil one".
Firstly, it should be noted that the doxology in the Sermon at the Temple (Matt 6:13 [cf. 3 Nephi 13:13]) is not a problem--indeed, biblical scholars such as Joachim Jeremias and early Christian texts such as the Didache show that Jesus would have uttered a doxology at the end of the Sermon on the Mount (see here and here).
Let us examine the other objections Case raise against the Book of Mormon:
Matthew 5:27//3 Nephi 12:27:
While the earliest Greek texts do lack the phrase τοῖς ἀρχαίοις, the meaning of the phrase is implicit in the Greek whether or not the phrase is original. This is because the parallel sayings in Matt 5:21 and 5:33 contain the phrase τοῖς ἀρχαίοις, so these words are understood in v.27 (via subtext), just as they are understood in vv. 38 and 43 where no Greek manuscript evidenced a need to repeat the obvious either. (John W. Welch, Illuminating the Sermon at the Temple & Sermon on the Mount [Provo, UT.: Foundation for Ancient Research and Mormon Studies, 1999], 202).
Matt 6:4, 6, 18//3 Nephi 13:4, 6, 18
While the extent textual evidence does support the term "openly" (ἐν τῷ φανερῷ) was added later, the only possible meaning of these verses is that God will openly reward the righteous with treasures in heaven at the final judgment. This understanding is sustained by the Greek verb for "reward" (αποδιδωμι). It has a wide variety of meanings, including "to give retribution, reward, or punishment." Its prefix απο can mean among other things, "out from." For example, in the word "apocalypse," the prefix απο means "out from" that which is hidden. In the verb αποδιδωμι, it may convey the idea of being rewarded απο, that is, "out from," the obscurity of the acts of themselves, or openly. Thus, one does not need the phrase ἐν τῷ φανερῷ in order to understand that "he who sees in secret will reward you απο, openly" (ibid., 205).
As we see, the arguments forwarded by Case do not hold any water. Such is typical of many of the criticisms against the antiquity and authenticity of the Book of Mormon.