If the Church could not teach truth back then, and for all time, than isn't the church worthless, unreliable and useless as a moral teacher?

I came from atheism, to deism and then to the Christian faith in some ways due to a search for objective truth. "I am the way the truth and the light"  and the Matthew chapter 16 15-20 Is an utterly false statement if the Church of Christ could fall into catechetical error  over so central in human life in communion with God that marriage is regarded as a sacrament where according to the Church grace is given. 

When exactly did the Church turn to error, and in what degree, there is not one place in either tradition and scripture that advocates for remarriage.