According to a number of critics, the earliest LDS theology was that of Modalism. One of the most commonly cited texts purportedly teaching this doctrine outside of Mosiah 15 in the Book of Mormon (see here for an analysis) is that of JST Luke 10:23 (KJV Luke 10:22). In the 1866-1867 RLDS Committee Manuscript, the text is rendered:
<23> All things are delivered to me of my Father; & <and> no man knoweth that the Son is the Father, & <and> the Father is the Son, but him to whom the Son will <reveal it.>
The 1867 RLDS and subsequent readings of the verse in the printed texts read:
All things are delivered to me of my Father; and no man knoweth that the Son is the Father, and the Father is the Son, but him to whom the Son will reveal it.
The argument is that this verse in the JST presents the person of the Son being numerically identical to the person of the Father. However, this is a prime example of absolutizing a single verse and wrenching it out of its immediate context.
In the previous verse(!), the text reads:
In that hour Jesus rejoiced in spirit, and said, I thank thee, O Father, Lord of heaven and earth, that thou hast hid these things from them who think they are wise and prudent, and hast revealed them unto babes; even so, Father, for it seemed good in thy sight.
In JST Luke 10:22, Jesus addresses the person of the Father, and is clearly differentiating his person from that of the Father, something that is inconsistent with Modalism wherein the Father and the Son are one and the same person.
Furthermore, even in v. 23, there is a differentiation between the two persons, as the Father delivers “all things” to the person of Jesus Christ; again, a distinction between persons.
Furthermore, when one examines the entirety of Joseph Smith’s revelations, one finds that the JST does not support Modalism; for instance, in classical texts that have been used for centuries to refute Modalism (e.g, Matt 3:16-17; 17:1-8; John 17; Acts 7:55-56, etc) the constant differentiations between the persons of the Son and the Father were never edited to “smooth things” over and identify the persons of the Father with the Son.
In short, the charge that JST Luke 10:23 teaches Modalism is yet another example of eisegesis by certain critics of the Latter-day Saint faith.