Understanding doctrine = knowing and understanding the Gospel.
And yet still not the means of salvation.
The thief on the cross almost certainly did not understand doctrine. Little children do not understand doctrine, and yet Jesus says we should be like them (in the sense of trusting Him) if we want to enter the Kingdom of God. (Matt. 18)
Don't misunderstand: doctrine is a good and necessary thing. It's that body of principles and beliefs by which we can maintain a consistent understanding of what the Gospels say, and for churches to maintain a consistent body of teaching.
Even so, doctrine is not fixed, even among bodies of people that we both would agree are Christians. One expects this, as doctrine is the work of men, trying to make sense of what the Gospels are all about.