Many Evangelical critics of the Church often criticise the respect and honour given to the prophet Joseph Smith by Latter-day Saints. I discussed this issue in some detail in my article:
With respect to Brigham Young’s high view of Joseph (who was also a close personal friend of his), the Tanners write:
The importance of Joseph Smith in Mormon theology cannot be overemphasized. Brigham Young, the second President of the Mormon Church, made these statements . . .
He is the man through whom God has spoken . . . yet I would not like to call him a saviour, though in a certain capacity he was a God to us, and is to the nations of the earth, and will continue to be. ([Journal of Discourses] Vol. 8, page 321). (Jerald and Sandra Tanner, Mormonism: Shadow or Reality? [5th ed.; Salt Lake City: Utah Lighthouse Ministry, 1987, 2008], 252; comment in square brackets added for clarification).
Daniel W. Bachman, in his excellent article showing the Tanners' blatant abuse of sources in Chapter 19 of their book, "Joseph Smith" (pp. 252-59), Mormonism–Shadow or Reality? History or Propaganda? Joseph Smith as a Case Study, discussed this particular text. He wrote:
The excerpt has been taken out of its context in Brigham’s speech. To begin with he is talking about Joseph’s role as prophet in foreseeing the Civil War and offering to save the nation from it. The sentence immediately following the Tanner excerpt shows that Brigham did not view Joseph Smith as a savior or God in the sense that Christ was, for he said, “He [Joseph Smith] was not the only-Begotten of the Father, who died for the sins of the world; but he was the Prophet of the Lord, through whom God spoke to the nations.” Brigham is here speaking in exactly the same sense as found in Exodus 4:16 where the Lord tells Moses that Aaron would be Moses’ spokesman to the people and Moses would be to Aaron “instead of God.” The readers of Shadow or Reality? know nothing of this. By divorcing these statements from their context, the meaning is changed subtly to suggest something that Brigham did not intend and which he specifically tried to interdict in his remarks.
Furthermore, while Brigham did not call Joseph a “saviour” in the above text, he did give a nod to it perhaps being an acceptable title for the prophet. While this might be shocking to some, especially those from Evangelical Protestantism, such is a perfectly valid title to use of mortals, not just deity.
In the Hebrew Bible, the title מושׁיע mošiac. While the term is never transliterated in the Bible, it appears 27 times in the Old Testament and means a "deliverer/saviour" and is used for both morals and deity.
Examples of mortals being called this title include:
And when the children of Israel cried unto the Lord, the Lord raised up a deliverer (מושׁיע) to the children of Israel, who delivered them, even Othniel the son of Kenaz, Caleb's younger brother . . . But when the children of Israel cried unto the Lord, the Lord raised them up a deliverer (מושׁיע), Ehud the son of Gera, a Benjamite, a man lefthanded: and by him the children of Israel sent a present unto Eglon the king of Moab. (Judg 3:9, 15)
And the Lord gave Israel Israel a saviour (מושׁיע), so that they went out from under the hand of the Syrians: and the children of Israel dwelt in their tents, as beforetime. (2 Kgs 13:5)
Therefore thou deliveredst them into the hand of their enemies, who vexed them: and in the time of their trouble, when they cried unto thee, thou heardst them from heaven; and according to thy manifold mercies thou gavest them saviours (מושׁיע), who saved them out of the hand of their enemies. (Neh 9:27)
And the saviours (מושׁיע) shall come up on mount Zion to judge the mount of Esau; and the kingdom shall be the Lord's. (Obad 1:21)
HALOT defines the term thusly:
4955 מוֹשִׁיעַ
מוֹשִׁיעַ: ישׁע pt. hif. > sbst.: מֹ(וֹ)שִׁ(י)עוֹ/עֲךָ, מוֹשִׁיעֶֽךָ/עֵךְ/עָם, מֹ(וֹ)שׁיעִים: deliverer, saviour:
—1. of people: a( pl. Ob 21 )Sept. pass., ? מוּשָׁעִים or (נוֹ׳ Neh 927; b( phrases וְאֵין מ׳ and simil., with no deliverer, helpless Dt 2227 2829.31 Ju 123 2S 2242 Is 4715; c) raised up by Yahweh for his people Ju 39.15 (properly referring to the “minor judges”, but carried over in Deuteronomic literature also to the heroic deliverers, Noth Überl. Stud. 49; Boecker 65; Soggin Königtum 133) 2K 135 Is 1920 4515;
—2. Yahweh as מוֹ׳: a( 1S 1019 Is 4515 638; b( with suffix 2S 223 Is 433 4926 6016 Jr 148 Ps 10621 or genitive Ps 711 )Israel( 177 (חֹסִים); c( no saviour but Yahweh 2S 2242/Ps 1842 Is 4311 4521 Hos 134. †
Therefore, there is no problem with Joseph Smith being called, in a limited sense, a "saviour."
Readers might find some parallels between both the term’s function as well as how it is transliterated with Book of Mormon Mosiah. Indeed, many LDS scholars (correctly, I believe) link Book of Mormon Mosiah with OT mošiac. For a discussion, see:
John W. Welch, "What was a Mosiah"?