Indefectibility (OSV Encyc. 1997)
Our Lord promised that he, with his divine power, would be with his Church always (Mt 28:18-20) and that evil or error would never overcome it (Mt 16:18). Good is stronger than evil; God is more powerful than the devil, although at times we may be tempted to give way to the opposite impression. God's thoughts are not how we tend to think, nor are his ways our ways (cf. Is 55:8). The triumph of Jesus Christ came through letting himself be put to death on the cross: humanly speaking it appeared as his failure.
God would not be all-powerful unless his power was greater than all the evil of the world put together. His mercy and love would not be infinite unless they were able to absorb and overcome all men's defects, wrong dispositions, and sins. It is God's specialty to turn evil to good.
We should therefore not be surprised to run into personal difficulties in our christian life, or to see the Church herself experiencing adversity, being the object of hostility, abuse and even persecution. "Blessed are you when men revile you and persecute you and utter all kinds of evil against you falsely on my account" (Mt 5:11). The Truth was crucified to save the world.
Certain points of faith become more basic in times of crisis, and need to be held onto more firmly. The Pope will never lead people astray in any teaching of salvation. The Church will never be deprived of our Lord's presence and protection. Nor will her mission be frustrated by the indifference or hostility of men. The Church will not fail: that is what we mean by the Church's indefectibility.
It is we, each one of us individually or several as a group together, who might fail. And we are likely to do so, if we do not maintain our supernatural outlook ("O man of little faith, why did you doubt?": Mt 14:31).
The result of temptations should be to increase our faith: faith in the power of God, the efficacy of the work of Jesus Christ, and the indefectibility of his Church. Thus, never losing our optimism, we will be able to help others overcome pessimism (or, worse still, barren criticism) which, by paralyzing Christians, blinds them to the needs of others and prevents them from putting their lives fully at the service of evangelization.