"Restorationism" is a strain in American church life.
Put briefly, restorationists teach that they are restoring apostolic or primitive Christianity. That somewhere along the way, the Church lost its way, and that it must be restored to that primitive purity. Restorationism encompasses a broad swath of American churches, from the United Church of Christ to the "churches of Christ" (the Campbellites), the Disciples of Christ. And though they wouldn't usually claim each other, Mormonism is from a historical standpoint the step-child of the Restorationist movement.
Jesus is clear: the gates of hell will not prevail against the Church (Matt. 16.18) For the Restorationist, the challenge is acknowledging the truth of what Jesus said with the claim that for some 1,500 years, the Church was not adequate, not fully the Church, or in serious, substantial error.
In a recent discussion, I pointed out that Lutheran catechisms sometimes query children, "When did the Lutheran Church begin?" with the answer given as "Oct. 31, 1517." And while a pastor argued that the proper answer should be April 25, 1530 (date of the adoption of the Augsburg confession), the answer should be -- if one properly believes that the Lutheran churches are nothing more than a continuation of primitive Christianity -- "the day of Pentecost."
Lutherans who believe there was a new beginning of the Church in 1517 (or 1530) have become early Restorationists. In other words, they are saying that there was no "church" (or that "church" only happened accidentally -- to use the philosophical term) from some point in early Christianity to 1517.
Of course, these Lutherans aren't the only ones falling for this error. Those who shill for the ordination of women argue that the Church -- by not ordaining women -- was erring -- or at least mistaken -- for centuries. Nothing changes if the argument is made that there were ordinations of women in the early churches: this is simply a variety of a Restorationist argument.
This is not a small matter. Either the Church was there -- in 203 a.d. and 568 and 1134 and 1516 and 2008 -- or it was not. Either there is -- now and then and always -- "one Lord, one faith, one baptism" (Eph. 4.5) or there was not. We can't have it both ways. It's very self-flattering to imagine that we know more than our Fathers in the faith knew, and that God needs us to do what He could not do. This is not to say that there have not been errors in the Church. That is plain. But God preserves His Church in love and pity. He does not need our tinkering to make that Church work.