Having to decide whether you believe that everything Joseph Smith taught is true or not is having to decide whether you should put your faith in Joseph Smith or not. If faith in Joseph Smith was necessary for salvation (read: being saved from the punishment of sin, i.e. forgiveness from sin), then everything Jesus Christ did on the cross is not fully valid.
2 Timothy 3:14-15 says:
"But continue thou in the things which thou has learned and hast been assured of, knowing of whom thou has learned them; And that from a child thou hast known the holy scriptures, which are able to make thee wise unto salvation through faith which is in Christ Jesus."
(Keep in mind that the holy scriptures that Paul was referring to could only be the Old Testament and four Gospels, because that was the only writings available to Timothy at the time this letter was written. No Joseph Smith, and therefore no Book of Mormon, Pearl of Great Price, or Doctrines & Covenants. Nevertheless, Paul assured Timothy he had all the wisdom necessary to lead to salvation, which is through faith in Christ Jesus alone.)
"Lord I lift these readers up to you today, that they can have confidence that YOU are who you say you are, and that they will be moved by your Holy Spirit to put their faith in YOU alone. Lord Jesus I know what foolishness it is to build our faith on the shifting foundation of man rather than on You, the solid rock. Thank you for continuing to show me that my faith is not unfounded; that you are as alive today as when you walked the earth. I anxiously await the day that we can be face to face in the throne room of heaven, so I can praise you and your goodness for eternity. Amen!"
