The Nature of God and Jesus

The vast majority of doctrinal differences between the LDS church and mainstream Christian churches have to do with the nature of God and Jesus. This is major a point of departure between mainstream Christianity and nearly all other groups that they might classify as not Christian because the fundamental nature of all understanding is based on these concepts.

For example,

Jesus and Satan are Spirit Brothers

Jesus is spoken of as the one and only, unique (μονογενες 3439) son of God (Jn 1:14, Jn 1:18, Jn 3:16, Jn 3:18, 1 Jo 4:9). There is no other like him (i.e. brother).

Jesus is God. He claimed to be God in word and deed. (Mk 14:61-62, Jo 5:18, Jo 8:58, Jo 10:30-33)

Jesus deserves to be worshiped as God. (He 1:6, Rev 5:11-14)

Jesus created Satan. (Col 1:15-16)

The LDS Church Believes in Continuing Revelation

Because the LDS church believes in continuing revelation, it might be difficult to nail down any particular doctrine as being in direct contradiction with the New Testament, because (at some level) it seems that any doctrine the church asserts to be true could change. They would probably characterize this continuing revelation as not something that goes back and makes editorial changes to fundamental doctrines, but there do seem to be some occasions when this has happened.

Blacks Prohibited from the Melchizedek Priesthood

Until 1978, blacks were restricted from holding the priesthood. This position was never codified, but it was universally applied.

In 1949, the First Presidency issued a statement:

The attitude of the Church with reference to Negroes remains as it has always stood. It is not a matter of the declaration of a policy but of direct commandment from the Lord, on which is founded the doctrine of the Church from the days of its organization, to the effect that Negroes may become members of the Church but that they are not entitled to the priesthood at the present time.

In 1978, President Spencer Kimball issued a statement:

a revelation had been received by President Spencer W. Kimball extending priesthood and temple blessings to all worthy male members of the Church.

In 1978, Bruce McConkie said:

Forget everything I have said, or what...Brigham Young...or whomsoever has said...that is contrary to the present revelation. We spoke with a limited understanding and without the light and knowledge that now has come into the world.

The church doesn't appear to have an official position regarding the origin of the priesthood ban or why God chose to institute it.

Plural Marriages

For example, plural marriages were once commanded (either to be practiced or, at least, accepted).

1843/1852 Revelation, D&C 132
For behold, I reveal unto you a new and an everlasting covenant; and if ye abide not that covenant, then are ye damned; for no one can reject this covenant and be permitted to enter into my glory.

In 1890, the president of the LDS church issued a statement regarding plural marriages.

And I now publicly declare that my advice to the Latter-day Saints is to refrain from contracting any marriage forbidden by the law of the land.

This is probably not the best example, because many(?) mainstream LDS adherents (not reformed or otherwise separate from the main body) will argue that polygamy was only ever allowable when God commanded it for specific individuals. They will point to Jacob 2:27,30 and 2 Sa 12:7-8 to make this claim.

In comparing doctrine, however, in the OT, having multiple wives was only permitted and never commanded, recommended (Israelite kings were told not to "multiply wives" De 17:17), or even encouraged (and the instances in which it was recorded as happening, having multiple wives was always a source of problems: Leah and Rachel, Hannah, David's sons, Solomon, etc.). In the NT, having multiple wives was expressly forbidden to leaders in the church (1 Ti 3:2,12, Ti 1:6), and it was never, at any time, endorsed as a way to behave. Rather, all talk of marriage (WRT believers) assumes only a single wife (1 Co 7, Lk 18:29, Lk 20:28, 1 Co 9:5, Eph 5:28).

As I understand what is taught in the Bible, having multiple wives was never something a Christian should have expected to be a part of (unless he became a Christian after his marriages). The LDS doctrine is at least compatible with the concept of this not being the case.

Mainstream Christianity Rejects the Idea of New General Revelation

The point is that the official LDS church position on a matter can change.

The authors of the New Testament always seem to be of the same mind that what you've been given is sufficient. There is no expectation of something more or anything new to come. John says:

John 20:30-31 (NASB)
Therefore many other signs Jesus also performed in the presence of the disciples, which are not written in this book; but these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name.

as if what he's written is enough. Paul says:

Galatians 1:9 (NASB)
As we have said before, so I say again now, if any man is preaching to you a gospel contrary to what you received, he is to be accursed!

There is a clear expectation we have been given everything we need and that we should not expect anything new to be added later.

The concept of continuing revelation might be seen as a fundamental doctrinal problem between the LDS church and other Christian groups' interpretation of the Bible. The fact that the LDS church will prefer the Book of Mormon to any tangible copy of the Bible (claiming that modern translations are flawed or are taken from corrupt sources) is also a fundamental doctrinal difference. The two groups have different textual sources of authority, and these sources are not always in harmony.

References

• Mormonism and racial issues/Blacks and the priesthood
• Do Mormons practice polygamy?
• Mormonism and the Question of Truth
• Is Jesus Christ the Spirit Brother of Satan?