The thing that is weird about the mainline Protestant churches is that the doctrine is dependent on the pastor. I'm mostly familiar with the United Methodists. One pastor can be very Bible-centered and another be preaching woo-woo theology--based on his/her ideas with scant attention paid to the Bible. I guess it IS like the Seven Churches of Revelation. There was the church of Philadelphia and then there was the church of Thyatira.
That is why preachers and teachers have a heavy responsibility to preach and teach Truth. "God is not a man that He should lie." His truth is found in the pages of Scripture--which is totally inspired by the Holy Spirit--the "Spirit of truth". Jesus affirmed ALL of the OT in John 10:35. The New Testament is affirmed by the witness of those Jewish men who became followers of Yeshua ha Maschiach (Jesus the Messiah of Israel). They went to their martyrdom gladly--counting it a blessing to die for their faith in Him. But even if they had not been martyred, they would have understood that to add to God's word with untruths would consign them to eternal damnation. We can totally trust His word. Too bad the Johnny-come-lately pastors from the apostate seminaries don't understand that.