I have a friend who is questioning the validity of Vatican II and here and here is the conclusion I came to if one embraces this position and carries it out to its logical conclusion. Please critique if the logic is flawed:
- If one rejects Vatican II, then one cannot trust the veracity of all previous Ecumenical Councils. What makes a council Ecumenical? We can’t say that it is Ecumenical if all of the Bishops in the world are there because this has never happened. We can’t say it is Ecumenical if all of the local Churches are represented because this never happened. We can only say a council is Ecumenical if the Pope ratifies it or at least accepts it as such, which has is the case with Vatican II.
- If one cannot trust all previous Ecumenical Councils, which would include the First Council of Nicaea, then we cannot know who Jesus was. Was He fully God and Fully man (the Nicene position) or was he only God (the Docetist view) or was He only a man (the Ebionite view)?
- If we cannot know who Jesus really was then we cannot know if He was really the Messiah.
- If we cannot know that Jesus is the Messiah then we cannot trust the Old Testament since it declared that the Messiah would come before Judah lost the scepter (Genesis 49:10), which was right around the time of the birth of Jesus. If my interpretation of Gen 49:10 is flawed then, without an infallible Magisterium to determine whose interpretation is correct, we still can’t trust the Old Testament since it may yield multiple opposing interpretations, any of which are viable options since it boils down to one’s own opinion as to what the text means.
- If we cannot trust the Old Testament then we cannot claim that the One True God has revealed Himself to humanity.
- If God has not revealed Himself to humanity then we cannot know who God is, as Thomas Aquinas demonstrated by natural reason, and therefore we must become Deists.
Ad Gloriam Ecclesiae 
For the sake of argument, how about a counter-example? The Coptic Orthodox deny 4 of the first 7 Ecumenical Councils whereas the Catholics and mainstream Orthodox accept all of the first 7. The seventh Council dealt with the acceptability of using and venerating holy images. If the Coptic Orthodox deny the Council which sanctions the veneration of icons does that mean they reject the veneration of icons?
Good point! It would not mean that they reject the veneration of icons, but it would mean they don’t have an infallible authority to guarantee they are not in error by venerating icons. This would result in their position being no more authoritative than the iconoclast position, which means one cannot really know whether iconoclasm or iconodulism is correct, resulting in an agnostic view on the matter. Likewise, one who rejects Vatican II may still embrace much of what the Catholic Church teaches but it would seem they don’t have a leg to stand on since they have rejected an infallible magisterium to back up their views. This would result in their position being no more authoritative than Protestants. Thus, they would have to end up becoming agnostic since they don’t have divine assurance that their beliefs are correct. What think ye?