Perhaps one of the most significant and moving passages of scripture is that of Jesus' great intercessory prayer found in John 17. One obvious fact here is that Jesus is not praying to himself. There are two "persons" involved. All Christians, LDS and Traditional, agree with this. Of course, many from both camps confuse the Trinity for Modalism, the belief that God is a single person who manifests himself in three different persons or modes, when in fact we all believe (or ought to believe) that Father, Son, and Holy Spirit are three distinct persons.
Some people I've talked with wrongly believe they're all the same person and thus mistakenly assume that Jesus is somehow praying to himself. Technically, this is a "heresy", and there are indeed two persons involved in this prayer--Father and Son. The obvious conclusion is that since there are two distinct "persons" there is (consequentially) more than "one".
Now contrary to LDS understanding, where each "person" is their own being (1 person per being!), Trinitarians believe that each of these three divine persons are actually one ontological substance, or one Being. (Or in other words, that God is one Being eternally existing in three persons, or that these three persons are ontologically one, meaning at the level of being.) This is their best explanation for the biblical data that God is three, but that God is also one. Although I don't subscribe to that particular solution, I'm not interested in mocking someone else for believing that way. "We claim the privilege of worshiping Almighty God according to the dictates of our own conscience, and allow all men the same privilege, let them worship how, where, or what they may" (11th Article of Faith).
However, lately I've encountered several [Christian] folk who seem to have made it their personal ministry in life to convince the Mormons of the error of their ways and who have set up blogs in the sole hope that our "misguided" eyes will be opened. After all, we're all "blind" because we're being "lied" to! Convinced their interpretation is the right one, these friendly folks bring up scriptures in the Old Testament, like Isaiah 45:5 ("I am the Lord, and there is none else, there is no God beside me"), Isaiah 44:6 ("I am the first, and I am the last; and beside me there is no God"), or Isaiah 43:10 ("before me there was no God formed, neither shall there be after me") which, they thus claim, somehow provides "evidence" that the Mormons are wrong.
However, I think it would be wise for them to recognize this is the same kind of reasoning that Jews would use against Christian belief. You see, Jews don't consider Trinitarians strict monotheists either. Jews only believe that God is one, without subscribing to the belief that God is three and one. In this sense, Trinitarian Christians and LDS Christians are in the same boat--we both desire to preserve the distinction among the three persons. (Trinity is Tri-Unity, meaning a unity and a plurality.)
Naturally, we come to different conclusions about how God is both one and three. Jews, on the other hand, don't believe in three divine persons (Father, Son, and Holy Ghost)--just one God. Christians believe that there are three divine and distinct persons, each of whom is fully God. Joseph Smith referred to the three divine persons as three Gods, thus Joseph Smith offered a robust meaning of the word person, affirming the threeness of God. (Christians may feel Mormons somehow compromise the unity of God in believing God is three beings, but this is simply wrong just as it would be for Jews to accuse Christians of violating the Bible because of their beliefs.)
A Christian might say that God is one but merely manifesting himself as three different persons, but that is actually Modalism, when in fact they are three distinct persons. By any means, I don't see any biblical evidence that God is ontologically one. I think Jews might want to convince a Christian that there are not three divine persons--but only one--by using those same Hebrew Bible/Old Testament verses.
New Testament verses that clearly state that God is one do not say God is "ontologically" one. It's just stated that they are "one". The Bible also says that husband and wife are to be "one"--in fact "one flesh"--but we don't interpret that to mean that husband and wife are somehow supposed to be one being. Now don't get me wrong. I'm not trying to over-emphasize the separateness of the Father, Son, and Holy Spirit, because I believe the Godhead to be infinitely more united as "one" than they are separate--one in every conceivable way except ontologically. But I don’t believe the only way to understand God’s oneness is to understand God is ontologically one. There are more ways to understand “oneness” than ontologically or numerically.
Christ prayed in John 17:11 for His disciples "so that they may be one as we are one" (New International Version). The King James version says "that they may be one, as we are." The New Living Translation puts it like this: "united just as we are". Obviously, this is not inferring that we are all supposed to become one substance or being--but one in terms of relationship, unity, and love. This is more in line with how I view the unity of the Godhead. Jesus wants us to be one with Him and Father--in exactly the same way--just as He and His Father are one.