I would say that the weakening of Christian values is present throughout the first stanza of the poem.  Yeats' construction of the world is one in which the unifying principles previously held are no longer in evidence.  The ideas abound in fragmentation, almost becoming a direct statement against the unity and coherence found in Christianity.  For example, the ideas of "the center cannot hold" and "the best lack all conviction, while the worst are filled with passionate intensity" are recognitions that Christian values are weakening.  The vision of the first stanza is of a world where there is little sense of hope and redemption that is such a part of the embrace in Christian values.  Yeats is able to clearly bring this out in his work and help to set the world of the first stanza as one in which the weakening of Christian values is the reality in which these conditions arise.  At the same time, the emergence of values that reflect a weakening of Christian notions of the good is evident in the second stanza in terms of the rise of the anti- Christ figure.  The "vision" in the second stanza is a direct reality of how Christian values have weakened as this figure "slouches towards Bethlehem."  The condition in which this figure emerges and the fact that this figure is not a direct embodiment of Christian values reflect a world in which these values have weakened.