This is slightly off thread (maybe not), but in my discussions with my LDS brothers and sisters, the topic of ‘The Great Apostasy’ always comes up. Being a member of the original Christian church, I have always wanted to know what it the Church taught that was the apostasy that kick separated us from Christ.
What was it?
Much of my wifes family is LDS and I always have wanted to ask, but family being family, I didn’t was to say something to accidentally give offense. This is a safe place.

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Fermat,
You have asked a fairly complex question, and it has been discussed before on other threads, so here are some basic building blocks for thinking about this:
(1) God has brought the word of the Lord to the people of His covenants through prophets or prophetesses. If one reads the Old Testament, one finds that many times the people are reprimanded because they have strayed from the basic laws, ordinances and covenants that God had established, and they are asked to return to Him. Here are example texts:
Isaiah 24:5 The earth also is defiled under the inhabitants thereof; because they have transgressed the laws, changed the ordinance, broken the everlasting covenant.
Amos 8:11 Behold, the days come, saith the Lord God, that I will send a famine in the land, not a famine of bread, nor a thirst for water, but of hearing the words of the Lord:
12 And they shall wander from sea to sea, and from the north even to the east, they shall run to and fro to seek the word of the Lord, and shall not find it.
Hosea 2:17 For I will take away the names of Baalim out of her mouth, and they shall no more be remembered by their name.
18 And in that day will I make a covenant for them with the beasts of the field, and with the fowls of heaven, and with the creeping things of the ground: and I will break the bow and the sword and the battle out of the earth, and will make them to lie down safely.
19 And I will betroth thee unto me for ever; yea, I will betroth thee unto me in righteousness, and in judgment, and in lovingkindness, and in mercies.
20 I will even betroth thee unto me in faithfulness: and thou shalt know the Lord.
21 And it shall come to pass in that day, I will hear, saith the Lord, I will hear the heavens, and they shall hear the earth;
22 And the earth shall hear the corn, and the wine, and the oil; and they shall hear Jezreel.
23 And I will sow her unto me in the earth; and I will have mercy upon her that had not obtained mercy; and I will say to them which were not my people, Thou art my people; and they shall say, Thou art my God.
(2) It is clear in reading the epistles of Paul, Peter, and James that as the New Covenant gospel was being preached and people were being baptized and leaders were being called in many places, Paul and Peter and James were concerned about wrong ideas that started to be taught and believed by some of the people (not all of them). Example texts:
Galations 1: …unto the churches of Galatia:
3 Grace be to you and peace from God the Father, and from our Lord Jesus Christ,
4 Who gave himself for our sins, that he might deliver us from this present evil world, according to the will of God and our Father:
5 To whom be glory for ever and ever. Amen.
6 I marvel that ye are so soon removed from him that called you into the grace of Christ unto another gospel:
7 Which is not another; but there be some that trouble you, and would pervert the gospel of Christ.
1 Corinthians 1: 10 Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment.
11 For it hath been declared unto me of you, my brethren, by them which are of the house of Chloe, that there are contentions among you.
1 Peter 5:8 Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour:
2 Peter 2:1 But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.
2 And many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of.
3 And through covetousness shall they with feigned words make merchandise of you:
James 3:10 …My brethren, these things ought not so to be.
11 Doth a fountain send forth at the same place sweet water and bitter?
12 Can the fig tree, my brethren, bear olive berries? either a vine, figs? so can no fountain both yield salt water and fresh.
13 Who is a wise man and endued with knowledge among you? let him shew out of a good conversation his works with meekness of wisdom.
14 But if ye have bitter envying and strife in your hearts, glory not, and lie not against the truth.
15 This wisdom descendeth not from above, but is earthly, sensual, devilish.
16 For where envying and strife is, there is confusion and every evil work.
The apostles gave it their all, and tried the absolute best they could to keep all of the doctrines pure and to keep the people seeking the Spirit, which meant they would all be having the guidance of personal revelation and would have the kind of charity and good works and no spirit of contention that Paul and James wrote about so much.
But just as in Old Testament times, the people strayed faster than the leaders could keep them settled in the pure gospel. As time went on, the leaders and teachers strayed also, so the doctrines of the pure gospel have changed over time and now there are all of the disagreements we find among churches who all follow Biblical teachings but don’t agree on those teachings.