The 1906 LDS edition replaced the preposition to with of here in Mosiah 3:19 (giving “an enemy of God” rather than the original “an enemy to God”). Subsequent LDS editions have continued with the original to. The text clearly prefers the preposition to after an enemy (nine times), including the first four cases listed below where king Benjamin himself is the speaker:
For the second example in Mosiah 16:5, the 1841 British edition replaced to with of (just like the 1906 LDS edition did in Mosiah 3:19). This second instance of the change to of continued in the LDS text until the 1920 edition. See the discussion under Mosiah 16:5.
There is also one example with the preposition unto: “for the devil is an enemy unto God” (Moroni 7:12), but there are no instances where of is used after an enemy. On the other hand, if the indefinite article an is replaced by the definite article the, we get only the preposition of:
Thus the earliest text consistently distinguishes between “an enemy (un)to X” and “the enemy of X”. The critical text will maintain this distinction.
Summary: Maintain the phrase “an enemy to God” wherever it is supported in the earliest text, including here in Mosiah 3:19.