According to LDS, what principles of the gospel were corrupted in the Great Apostasy?

Within Mormon (LDS) theology, you can't speak of Church organization without speaking of priesthood authority. The Priesthood exists without the Church but the Church cannot exist without the Priesthood. The Priesthood is the power and authority given by God to act in the name of Jesus Christ. So it makes no sense to a person of the LDS faith to separate the two.

But the question was what specific principles of the gospel were corrupted. I would have to begin with the idea that God the Father and Jesus the Christ are of one substance (h*m*ousia) which was cemented for Christiandom by the Nicene Creed (first council of Nicea in 325). That is a major departing point from what a Mormon would believe is a correct principle of the Gospel. Mormons believe that God the Father, Jesus Christ and the Holy Ghost (Spirit) are three distinct beings with one purpose. Jesus Christ and the Holy Ghost are not the same being as God the Father, who is our only God.

Another difference that is more subtle is in regards to soteriology. While Mormons agree that all humankind is freely given grace by the death and resurrection of Jesus Christ (the atonement) as do most Christians, Mormons believe that obedience to the laws and ordinances (saving works done under priesthood authority, such as baptism) of the gospel is also required. Some Christians would say that God approaches humans with God's saving works and never the other way around. Mormons believe that some effort is required by humans to receive this approach (free will and choice). This does not change the fact that Jesus Christ is the bridge between God and humankind and nobody can return to the Father any other way.

Other important principles that have experienced a slow departure over the years of the Church would include the following ideas, not all of which are opposed by all denominations of Christians. Humans are not born sinners because of the Fall of Adam, and thus are judged for their own sins. Mormons do not feel that babies require baptism. Baptism is required through proper Priesthood authority when a person is capable of understanding and accounting for their own choices. And finally, and probably a major point of irritation to many, the words of the Bible are not the final words of God. Prophecy and revelation are still alive and present in the world. God has not finished speaking to humans.

This is not all-inclusive, and I'm sure there are many more points that are perfectly good and important, but this is the list that expresses the areas of greatest concern among my fellows at the (Presbyterian) seminary I attended.

There are dozens upon dozen of examples of parts of the Gospel of Christ that were corrupted. A few:

• Agency -- Man has complete choice over his eternal destiny -- whether to follow Christ, or whether to follow the devil.

• Apostles -- The pattern of 12 apostles, or special witnesses of Christ, is key to Christ's church.

• Baptism -- Baptism is by immersion and for accountable individuals, not of infants, nor by sprinkling

• Eternal marriage -- This ordinance is central to God's plan. Families bound by eternal marriage will persist after this life.

• Nature of God -- The Godhead is three distinct but united persons: the Father, the Son, and the Holy Ghost. The Father and the Son have bodies of flesh and bone.

• Nature of Man -- Man has existed since before the creation of the world. He is the literal spirit child of God the Father.

• Ordinances -- certain rites combined with covenants, e.g. baptism, are required to return to God

• Ordinances for the deceased -- the living perform essential ordinances in proxy for the dead

• Prayer -- Man prays directly to God in the name of Christ, not through priests or saints.

• Priesthood -- To performance ordinances that are honored by God, man must possess the literal power to enact in heaven that which is done on earth (Matthew 18:18)

• Revelation -- God did not stop speaking in 70 AD. He will continue to reveal many great an important things pertaining to the kingdom of God.

• Spirits -- Man has a spirit distinct from his physical body. At death, these two separate.

• Temples -- These holy structures are extremely important, both for revelation and to receive promised blessings

Many of these are believe by other Christians; e.g. not all Christians believe predestination, or the Trinity. These principles were not all necessarily wholly lost, but there were at some point or another corrupted by a large number of people.

I've had a couple of conversations with LDS...where they have sought to emphasize the broad agreement with essential beliefs with other Christians. I find it hard to reconcile this with the LDS doctrine about the Great Apostasy and am trying to understand what they really meant.

Broadly speaking, much of the Great Apostasy consisted of loses or ommissions of principles of the Gospel, rather than outright perversions.

...there are many plain and precious things taken away from the book, which is the book of the Lamb of God.

And after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles...because of the many plain and precious things which have been taken out of the book, which were plain unto the understanding of the children of men, according to the plainness which is in the Lamb of God — because of these things which are taken away out of the gospel of the Lamb, an exceedingly great many do stumble, yea, insomuch that Satan hath great power over them.

1 Nephi 13:28-29

The missionaries may have been trying to convey that non-LDS Christianity is not so much wrong, as it is incomplete.

I should point out that the Great Apostasy was not just about loss of knowledge. It was just as much about loss of authority. No longer did a man on the earth have the power to say "Thus saith the Lord", to speak scripture, to hold the "keys of the kingdom of heaven", and to enact in heaven that which was done on earth.

So regardless of the number of common beliefs between LDS and many other Christians -- even if there existed a group that believed essentially every Mormon doctrine -- they would still be different in that the LDS Church has the (only) 12 Apostles. The LDS Church has Peter; the other has the sons of Sceva (Acts 19:13-16).

The answer to this question is by no means short or simple. A great many principles were corrupted over the course of almost 1,700 years from the time Christ's apostles were martyred and Joseph Smith was called to restore the Church.

However, I would say that the most significant principle which was corrupted very early in the great apostasy was the identity of God. The restored Gospel teaches us that we are very literally the spiritual offspring of God, our Father in Heaven.

I have said, Ye are gods; and all of you are children of the most High. (Psalms 82:6)

The Spirit itself beareth witness with our spirit, that we are the children of God: And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together. (Romans 8:16)

Just as I am a father, and desire more than anything for my children to grow and inherit every happiness I can offer them, so too does our Father in Heaven desire for us all to achieve eternal happiness, and become exalted as he is. He wants us to become Gods, He wants us to be like him.

For behold, this is my work and my glory—to bring to pass the immortality and eternal life of man. (Moses 1:39)

By changing this single principle, the entire gospel of Jesus Christ is corrupted, corrupted to the point that any return to the truth is considered blasphemy by the majority of "mainstream" Christianity, which is exactly what the Devil wants. Satan's design is to keep people away from the truth by making them believe that the true, uncorrupted priciples of the gospel are evil.

Entire books have been written detailing how the world fell into apostasy and what was lost, like James E. Talmage's book, "The Great Apostasy". There's also a Wikipedia article that does a descent job of detailing some of the other major principles which were corrupted.