Why are Gothic cathedrals considered the ultimate expression of the Age of Faith?The High Middle Ages

There are at least two ways in which this is true.

First, although this was the Age of Faith, it was also an age of growing scientific knowledge.  Gothic cathedrals celebrated that knowledge.  The knowledge of engineering was celebrated through things like the flying buttresses.  The knowledge of geometry was used to lay out the cathedrals’ floor plans. 

Second, all of this was used for the glory of God.  The knowledge was not applied to secular buildings but to churches.  The engineering knowledge was being used to draw people’s minds towards God.  Because these cathedrals used scientific knowledge (and huge amounts of effort) to glorify God, they are the ultimate expression of their era.