11 The Catholic Church thus affirms that during the two thousand years of her history she has been preserved in unity, with all the means with which God wishes to endow his Church, and this despite the often grave crises which have shaken her, the infidelity of some of her ministers, and the faults into which her members daily fall. The Catholic Church knows that, by virtue of the strength which comes to her from the Spirit, the weaknesses, mediocrity, sins and at times the betrayals of some of her children cannot destroy what God has bestowed on her as part of his plan of grace. Moreover, "the powers of death shall not prevail against it" ( Mt 16:18). Even so, the Catholic Church does not forget that many among her members cause God's plan to be discernible only with difficulty. Speaking of the lack of unity among Christians, the Decree on Ecumenism does not ignore the fact that "people of both sides were to blame", and acknowledges that responsibility cannot be attributed only to the "other side". By God's grace, however, neither what belongs to the structure of the Church of Christ nor that communion which still exists with the other Churches and Ecclesial Communities has been destroyed. Indeed, the elements of sanctification and truth present in the other Christian Communities, in a degree which varies from one to the other, constitute the objective basis of the communion, albeit imperfect, which exists between them and the Catholic Church. To the extent that these elements are found in other Christian Communities, the one Church of Christ is effectively present in them. For this reason the Second Vatican Council speaks of a certain, though imperfect communion. The Dogmatic Constitution Lumen Gentium stresses that the Catholic Church "recognizes that in many ways she is linked" with these Communities by a true union in the Holy Spirit.
Source: Ut Unum Sint (Vatican.va)