Textual: Abinadi is citing Exodus with only minor changes:
Ex. 20:2-4
2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage.
3 Thou shalt have no other gods before me.
4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: (the words in italics are emphasized to note the locations of the differences between Abinadi and the KJV text).
The deletion of "that is" is a simplification procedure. This is a case where a change is made that is not related to italicized words in the KJV (Book of Mormon Critical Text. FARMS 1987, p. 431). There is very little specific that can be gleaned from this change, other than the confirmation that the KJV text was not copied directly and completely faithfully.
The more interesting change is the shift from the plural "gods" of the KJV to the "God" of Abinadi. This may be a simple change similar to the reduction of the "that is" clauses, a simple shortening of the text. However, it is also possible that in the context of Abinadi before the priests of Noah.
To understand this possible context, we must remember that the priests of Noah were synthesizing the old religion with elements of the new religion. One of the fundamental principles of the law of Moses was the belief in the One God. That belief is so fundamental that it would be impossible to alter in a single generation from Zeniff to Noah (even assuming that there were converts from polytheistic cultures among them - the converts might be accustomed to multiple gods, but would have willingly given them up. Such people tend to be even more protective of their new religious principles). It is therefore likely that the priests of Noah had not mixed in new gods. What they had done was to so alter the law as to effectively deny the one.
This is Abinadi's accusation. Abinadi may have very consciously recited the verse with God in the singular because it would be easy for the priests to deny that they worshipped multiple gods. However, the accusation isn't in multiple gods, but rather that they have perverted the worship of the One God.
Mosiah 12:37
37 Now Abinadi said unto them, Have ye done all this? I say unto you, Nay, ye have not. And have ye taught this people that they should do all these things? I say unto you, Nay, ye have not.
Abinadi forces home his point. They should believe in the One God who brought his children from bondage in Egypt and gave the law to Moses. Abinadi accuses them of denying that God. As we see in the next verse, this is tantamount to declaring the priests blasphemers, and they react accordingly.
Textual: The modern chapter break at this point is unfortunate. It does not exist in the 1830 edition, and this interruption creates too much conceptual distance between this remarkable accusation and the effect of that accusation upon the priests of Noah. This chapter should always be read with the next one for the sense of the power of the argument and drama of the situation.