When Christ established his Church he did clarify and revealed many doctrines, which we see them being mentioned in the Bible, but just not stated enough for everyone to understand them. That is the Bible we have in its present form. Many pieces of important and relevant doctrines got lost or corrupted over time.
Some evidence of these doctrines can be further found in some of the writings of early fathers and other ancient documents. For example doctrines such as Pre Mortal existence, degrees of glory, potential of man becoming gods, baptism for the dead, baptism by immersion, etc, which are not explicitly mentioned in the Bible, but we see them being brought up in other ancient sources. Some of these doctrines were obscured and suppressed with the apostasies, not only the apostasy of the early primitive church, but the Israelites apostasies. That is what happens in an apostasy, books get lost, doctrines altered, corrupted and new ones are added.
The LDS Church not only restored what was lost during the time of Christ, but even things from the Old Testament, because the Lord promised to restore everything (Acts 3:19-21) in this epoch, the last days.
19 Repent ye therefore, and be converted, that your sins may be blotted out, when the times of refreshing shall come from the presence of the Lord;
20 And he shall send Jesus Christ, which before was preached unto you:
21 Whom the heaven must receive until the times of restoration of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
Some of the doctrines I mentioned before were restored in these latter days: degrees of glory in the resurrection, Pre Mortal existence of men, baptism for the dead, potential of men becoming like their Father in heaven, baptism by immersion. Other things that they had in the past were revelations from God, ongoing cannon of scriptures, prophets, apostles, seventies, temples, etc.
These doctrines and priesthood authority were restored with celestial manifestations and visitations from prophets and apostles of old, including the visitation of the father and the son. Celestial messengers in these last days were not only seen and heard by Joseph Smith alone, but by many credible people. Some celestial manifestations were eye witnessed by hundreds on people.
The gospel was preached to every nation and creature and then the end came, meaning that the Church of Christ with its authority and priesthood was not longer recognized in heaven. What came afterwards was an earthly organization. See the scripture below.
And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come. Mat 24:14
See what Doctor James E. Talmage has to say about the interpretation of this verse.
It is claimed by many that the “end” referred to in the passage quoted is not necessarily the close of the final dispensation, not what is commonly spoken of as the end of the world, but the closing up of the gospel dispensation then current; and in support of this interpretation it is urged that following the utterance quoted Christ proceeded to predict the calamities then awaiting Jerusalem. That during the period covered by the earthly ministry of the apostles, the gospel was preached in all the civilized nations of the eastern hemisphere, is evident alike from scripture and from the uncanonical writings of repute relating to that period. Paul speaks of the gospel as having been carried in his day to all the world, and as having been preached to every creature under heaven.
The Great Apostasy, James E Talmage, p36
According to Paul, the gospel had been preached to everyone.
…This is the gospel that you heard and that has been proclaimed to every creature under heaven, and of which I, Paul, have become a servant. Col 1:23
Compare with Romans 10:18
But I say, Have they not heard? Yes verily, their sound went into all the earth, and their words unto the ends of the world. Rom 10:18
Then I remember the words of Christ…
Therefore say I unto you, the kingdom of God shall be taken from you, and given to a nation bringing forth the fruits thereof. Mat 21:43