True, Jesus Christ did in fact reveal his gospel when he came upon the earth in the 1st century A.D. But what you fail to understand is that after Christ came all the Apostles except John were murdered and the Church gradually fell into a state of apostasy. For a long time there was not a true Church of Jesus Christ upon the earth. Paul surely was an Apostle of the Lord Jesus Christ. Of this you speak truly and these were the words of Paul:
Ephesians 4:11-14
11 And he gave some, apostles; and some, prophets; and some, evangelists; and some,
pastors and
teachers;
12 For the
perfecting of the saints, for the work of the ministry, for the edifying of the body of Christ:
13 Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
14 That we henceforth be no more children, tossed to and fro, and carried about with every wind of doctrine, by the sleight of men, and cunning craftiness, whereby they lie in wait to deceive;
Here we read that in the true church of Jesus Christ, there would be Apostles, Prophets, Evangelists, Pastors, Teachers, etc who hold the priesthood of God to administer the gospel and do the work of the ministry for edifying of the body of Christ (which is his church). How long would these offices of the priesthood be needed in his church? Until we all come in the unity of the faith and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ. Let me ask you if we have come to that unity yet? If not, then wouldn't you expect there to be Apostles and Prophets in the true church of Jesus Christ? What church since the death of the Apostles has had 12 Apostles or prophets? Tell me which Catholic or Protestant church has had the true church with 12 Apostles and Prophets? They haven't and that is one big sign that the true church of Jesus Christ fell into a state of apostasy. The Great Apostasy that occurred gradually after the Apostles were murdered was prophesied by Amos the Prophet of the old testament:
Amos 8:11-12
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.
Amos lived approximately 811 B.C. This didn't occur before his days but was prophesied to come after. The Apostle Paul in his day warned of those who were to come and turn away the flock:
Acts 20:29-30
29 For I know this, that after my departing shall grievous wolves enter in among you, not sparing the flock.
30 Also of your own selves shall men arise, speaking perverse things, to draw away disciples after them.
Again Paul taught:
Ephesians 2:19–22
19 Now therefore ye are no more strangers and foreigners, but fellow citizens with the saints, and of the household of God;
20 And are built upon the foundation of the apostles and prophets, Jesus Christ himself being the chief corner stone;
21 In whom all the building fitly framed together groweth unto an holy temple in the Lord:
22 In whom ye also are builded together for an habitation of God through the Spirit.
2 Thessalonians 2:1–3
1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him,
2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand.
3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
Paul tells us here that there would come a falling away from the church. Here are other verses warning of the falling away:
Acts 12:1–3; 2 Timothy 4:3–4; 2 Peter 2:1–2.
John himself in his Revelation tells of the Great Apostasy from the church:
Revelation 12:5-6
5 And she brought forth a man child, who was to rule all nations with a rod of iron: and her child was caught up unto God, and to his throne.
6 And the woman fled into the wilderness, where she hath a place prepared of God, that they should feed her there a thousand two hundred and threescore days.
Here the woman represents the church of Jesus Christ and the man child who eventually was to rule all nations was Jesus himself. He was caught up unto God and to his throne. Then the woman fled into the wilderness where she had a place prepared of God. This is the church going away because of apostasy. It was to eventually be restored after a great deal of time.
The Apostle Peter prophesied of the church being restored to the earth after the great apostasy:
Acts 3:19–21
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 restitution of all things, which God hath spoken by the mouth of all his holy prophets since the world began.
Here we learn that there will be a time when Jesus Christ will restore all things. If we have had the gospel and the church of Jesus Christ since the death of all the Apostles, why would there be a need for the restitution of all things. That time was to come before Christ was to come again on the earth. When the time of the restitution of all things has come then Christ will in that time come again. So a restoration of the His gospel would come again on the earth by establishing his church again on the earth. True, much of the gospel has made it through the ages in the bible but the priesthood of God and his church need to again be reestablished in these latter days before the coming of our Lord and Savior happens again on this earth. The restitution of all things means that all things that Jesus finds important to restore will be restored in these latter days. In the church this time is also known as the dispensation of the fullness of times. Through the prophet Joseph Smith, the Lord has again restored his true church upon the face of the earth and it is again a church with Apostles and Prophets, etc.
Joseph Smith did not preach another gospel but restored the same gospel of Jesus Christ and his church upon the earth with all the authority of the priesthood to minister unto the body of Christ and to officiate in the ordinances of the gospel such as baptism and the laying on of hands for the gift of the Holy Ghost until we all come to the unity of the faith.