If you die believing in Jesus Christ as your Savior, and that belief has led you to repent of your sins and keep His commandments, loving God and doing good to others, being meek and humble and childlike, teaching all these things to your children, being a good example of holiness and righteousness to them, and to all, you will be found among the righteous in the next life. You will not be found among those who have to go to hell because they would not repent.
If you die believing in Christ, but that belief has not led you to repent of your sins or keep His commandments, are you really a believer?
Most Christians are somewhere between the two right now, and we will let God, who is both just and merciful, help them learn the lessons of life that they have not learned yet, and judge them when the time comes according to their individual behavior towards God and towards others. It is for us to exhort all to faith in Christ, repentance, holiness, righteousness, keeping the commandments, and so forth.
Now, you added something else. What if you believe in Christ, but reject those He sends to you to speak and act for Him?
Obviously, it would a serious thing to reject Peter, Paul, Isaiah, Moses, and Abraham since you know that Christ has sent them. It would be tough to convince anyone that you believe in Christ if you reject the words He spoke through Peter and Paul, who were sent to speak for Him.
When you come to know by the witness of the Holy Spirit that Jesus also sent apostles in your day, Joseph Smith and Brigham Young and their successors, that changes things. But right now, you don't have that witness, and right now, God does not hold you responsible for that.
He does hold you responsible for doing what you already know from the Bible that you should do, as summarized above -- study the Bible and live accordingly.
A vital part of that is receiving the apostles and prophets whom Jesus has sent, and being willing to receive whomever Jesus may choose to send, because you cannot claim that God can no longer speak to you, nor would you say that you will not hear if He does.
In other words, leaving your personal feelings against Joseph and Brigham out of it for the moment, God can do what He wants, and you are willing to let Him. He has raised up apostles and prophets in the past, to speak to His people, to organize them, to lead them, to direct their labors in the Gospel of Jesus Christ, to guide and direct the preaching of the Gospel of Jesus Christ. It is His way of doing things. He can do it again anytime.
Perilous times are prophesied before the Second Coming. The world has gone through much in the last 100 years, and surely there is more to come. The Lord said He wants to gather His people often. He does so by sending prophets and apostles to them:
37 O Jerusalem, Jerusalem, thou that killest the prophets, and stonest them which are sent unto thee, how often would I have gathered thy children together, even as a hen gathereth her chickens under her wings, and ye would not!
They probably said in their hearts that they were already gathered. Weren't they living right there in Jerusalem? But they did not receive those whom Jesus sent to them, nor did they receive Jesus Himself when He came in person, nor His apostles after His earthly misson was complete.
Can you say in your heart, that whatever God does, you will support it, so long as you are able to know that God is doing it? That is the purpose of the witness of the Holy Spirit. Ask of God, that you may know what God is doing today, and whom He has sent to lead, guide, and organize those labors, as the Second Coming draws nigh.