I think the "trick" to understanding Nietzsche's critique of group morality would be to link his idea of strength with the idea that one "is" through "becoming" - those who continue to subscribe to group morality, or who were historically complicit in its development, are weak because they will not subscribe to this idea of strength; the source of "strength" for Christian group morality is precisely its weakness, because the point of man's weakness is the point where God can display His grace towards man - it is necessary to know that one is weak, in order to know God's strength, therefore the strength of the Christian comes from outside of him or her ... to the Christian, Nietzsche's idea of strength as sufficient daring to consider strength and weakness in terms of the individual setting himself deliberately against group morality, does not really make sense. One could regard Nietzsche's interpretation of the strength/weakness axis as a deliberate subversion of the idea of strength and weakness inscribed in Christian group morality; for Nietzsche, strength is an attribution of the individual who sets himself against group morality, and weakness is subscription to an idea of strength which places Christian believers at the mercy of God who asks them to glorify in their "weakness", because that is how He can display His strength in their lives