Darth you are misquoting Whitmer. He stated his own opinions about how the church had apostasized after the alleged quote from God.
Darth J wrote:
...do you also believe his statement that God called him and gave him authority to start a new church because Joseph Smith was a fallen prophet?
Just a few points. Whitmer's opinions are nothing more than Book of Mormon doctrine, that's what he strove to align his beliefs to. As he states in his three different publications and numerous newspaper correspondences, etc., he was troubled by the following as they occurred:
a. When Joe gave a false revelation (Canada copyright selling).
b. When Joe changed published revelations (that David was present for).
c. When Joe removed "Jesus Christ" from the name of the church.
d. When Joe added High Priests.
e. When Joe attached polygamy to salvation (Mormon "exaltation").
Of course there were plenty more that David identified, but these five were in each of his three publications and are sufficient to cause any believer in Mormonism or the Holy Book of Mormon to pause.
Regarding Whitmer starting a "new" church and being given "authority" to do so is inaccurate. Whitmer argued that prior to 1830, there was a church in operation, at least as it was practiced in the NT and the new covenant Book of Mormon Church. "Church" is any group of believers or individuals as rightly taught by most Evangelical Christians.
In D&C 10, Jesus confirmed in the summer of 1828 that his church was already on the earth and the Book of Mormon would "establish" it. The word "establish" throws some people off. As used by Jesus throughout the Book of Mormon, it meant to strengthen that which already existed. David also rightly taught that the invisible church could only be entered one way and was something more between us, Christ, and believers.
Baptism was never into a denomination, only into Christ, which is true and why most protestant churches hold membership and baptism separate. They will respect baptisms performed by other denominations. This criticism was also given by Alexander Campbell to Sidney Rigdon.
Thus, David left Joe, returning to that simple gospel that existed from 1828 to 1830 and avoided the Book of Mormon condemnation of Gentiles who "build up churches to themselves" as Joe did. The tragedy of course is that the sole book designed by God to finish what the Reformers began, became sequestered to a single denomination, much like what happened to the Holy Bible. The same spirit that took hold of Romanism took hold of Joe, the fruit is the same: OT priesthood and high priests, sexual perversion, the baptism of children, improper use of the communion, claiming to the only true church, etc.