The more I read the Book of Mormon the more I found passages that agree with traditional Christianity more than it agrees with modern day Mormonism.
Here is one from Mosiah 27 that talks more about becoming a Son or Daughter of God AFTER you are born again.
24 For, said he, I have repented of my sins, and have been redeemed of the Lord; behold I am born of the Spirit.
25 And the Lord said unto me: Marvel not that all mankind, yea, men and women, all nations, kindreds, tongues and people, must be born again; yea, born of God, changed from their carnal and fallen state, to a state of righteousness, being redeemed of God, becoming his sons and daughters;
26 And thus they become new creatures; and unless they do this, they can in no wise inherit the kingdom of God.
Nowhere that I know of in the Book of Mormon teaches that we are all Sons and Daughters of God no matter what we believe. Over and Over in the Book of Mormon it teaches that you only become a Son or Daughter once you have accepted him.
This is one thing that really threw me off when I was a TBM. I was always confused why the Book of Mormon never talked about Baptisms for the Dead, the 3 degrees of Glory (it only talks of Heaven and Hell), Temples, etc. Also if you gave somebody a Book of Mormon and had them only read it they would walk away still thinking the Trinity was true with all three being one.
All these things came latter with the D&C and by studying the Lectures on Faith you can see that many early beliefs were very similar to Protestant beliefs.