“Do Mormons believe that Jesus is truly God?
The correct answer is that mormons believe that Jesus is “a” god, one among an uncountable multitude throughout the universe, though only one of three gods that comprise a ‘godhead’ for oversight of this planet.”
So do they believe that Jesus is the only way to salvation?
That is a two-part question Bear (in fact could go many other ways with it as well - is the mormon Jesus the same as the Biblical Jesus of Christianity). Part 1 is salvation is the free gift of resurrection that Christ's atonement provides to everyone (whether atheist or faithful believer in Christ) so that they may have their spirits and bodies reunited after death to appear before God for judgment. Remember, mormons believe in a 'heaven' with three levels. This permits ALL to at least be in on the lowest level of heaven - a type of universalism. These will not be in the heaven with the Father.
Part 2 of mormon salvation is salvation by "grace" (synonymous with exaltation to godhood). This is summarized out of the book of mormon -
2 Nephi 25:23 -"For we know that it is by grace that we are saved, after all we can do".
Basically, all their works, such as becoming a temple mormon, being baptized for the dead, being married and sealed in a temple, following the word of wisdom, doing other 'good works', etc earns them points (so to speak) on the advancement chart to becoming a god (males only) and attaining the highest level of the heavens. They are required by their own efforts to become perfect and sinless to be worthy of exaltation. Women can only become goddesses by the will of their husbands.
The matter becomes very complex in that their doctrines require perfection and sinlessness BEFORE "grace" can be obtained.
Now this brings to mind another point - probably highly debated by mormons - and that is that Joseph Smith has the final say upon those entering 'heaven' because of his exalted state of being the one to 'restore' 'true' Christianity to the world.
Finally, on the sin question, which is critical to the issue of salvation, mormon teaching also state that mormons can cleans themselves of sin and save themselves.
Confusing? It certainly is, so in summary mormons believe that salvation under Jesus permits ALL to be resurrected and then to progress on the basis of their works to godhood.
Christianity cuts through this, stating that we can be in heaven (there is only one) by grace (God's unmerited - meaning we don't have to work for it) favor through the act of faith in believing in the sacrifice and resurrection of Jesus as our Lord and Savior.