Apostasy is a time when people believe things that are not true. Throughout the Bible, there were times of apostasy. These led to God taking away the privilege of having a prophet to communicate God’s will to the world, leaving them to figure out truth for themselves. Always, however, God eventually returned to the world a prophet, to give them another chance to explore the truth.
After Jesus died, the apostles ran the church under the direction of Peter. They fought a constant battle against encroaching apostasy, even reporting that all of Asia had been lost to it. Through letters and visits, they attempted to straighten out misinterpretations of doctrine and situations where people simply openly inserted their own ideas for truth. They could do this because apostles are also prophets. Peter received a vision in which he learned that nothing God had chosen was unclean, leading to changes in practice. As a result of his revelation, the gospel was taken beyond the Jewish people and circumcision was no longer required. Peter’s vision demonstrates why it is impossible to properly operate God’s church without a prophet—situations change and someone must have the authority to speak for God in an official capacity. When a dispute arose over circumcision, Peter was able to resolve it by sharing his vision. Because he was recognized as the person authorized to receive official revelation, the practice was changed.
However, the apostles were eventually dead, except for those who were promised they would live until Jesus returned. God did not replace them because the world was too far in apostasy and too few people cared to know what was true. They preferred doing things their own way. It was too dangerous for the apostles, and so, God took the priesthood from the earth.
A small and brave group of Christians continued on, keeping Christianity alive, but they operated without authority from God. There was no priesthood and they had no prophets and apostles. The Bible says the church must be built on a foundation of prophets and apostles, and so, without those the fullness of the gospel could not exist.
Now therefore ye are no more strangers and foreigners, but fellowcitizens with the saints, and of the household of God; And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone (Ephesians 2:19-20)
The Christians did the best they could and Christianity survived through their valiant efforts. Inevitably, however, disputes arose over doctrine and interpretation of scripture. When these happened, there was not a prophet to resolve the problem and so each person had to decide for himself what was true. Various sects began to develop as those who disagreed simply started their own branch of Christianity in order to teach their preferred version of the doctrine. Councils were held in the fourth century in which truth was put to a vote. Those who disagreed with the majority were excommunicated and even more churches were started.
Today there are a multitude of Christian denominations. While they share a belief in Jesus Christ, they teach widely differing doctrines on essential topics. Some no longer teach the divinity of Jesus Christ. Some teach that baptism is not essential, while other teach that it is. Some teach we must keep the commandments and some consider the commandments to be suggestions. God is not a God of confusion and all these things cannot be true at the same time.
The only way to resolve the problem, of course, is by restoring prophets to the earth. God warned us of an apostasy:
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: 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. (Amos 8:11-12)
He also promised a restoration of all things:
And he shall send Jesus Christ, which before was preached unto you: Whom the heaven must receive until the times of restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began. (Acts 3)
Learn more about the restoration:
Joseph Smith, first prophet of the restoration