“We are still Latter Day Saints, all of us, even if we change some things or we break the rules or we have complete doubt that God exists.”
Why is it such a big deal when people “change some things”?
Nephi saw in vision our day, and the problems that were caused when changes were made to the plain gospel taught by Christ and his apostles. He was told,
“Wherefore, thou seest that after [the Bible] hath gone forth through the hands of the great and abominable church, that there are many plain and precious things taken away from the book, which is the book of the Lamb of God.
“And after these plain and precious things were taken away it goeth forth unto all the nations of the Gentiles… because of the many plain and precious things which have been taken out of the book, which were plain unto the understanding of the children of men, according to the plainness which is in the Lamb of God—because of these things which are taken away out of the gospel of the Lamb, an exceedingly great many do stumble, yea, insomuch that Satan hath great power over them” (1 Nephi 13:28-29).
The Savior has given us his gospel so that we can be happy in this life, and qualify for eternal life in the life to come. Any changes to that gospel will make us “stumble” on that path, and perhaps even lose our way and prevent us from receiving either of those two gifts.