This same decree presents God's solution to this situation, the ecumenical movement:
Ecumenism, defined as "worldwide Christian unity or cooperation", is not a new subject to Christianity, as the idea of a universal (catholic) church dates back to the times of the New Testament. Jesus Christ referred constantly to the unity His church should have:
"I pray not only for them, but also for those who will believe in me through their word, so that they may all be one, as you, Father, are in me and I in you, that they also may be in us, that the world may believe that you sent me. And I have given them the glory you gave me, so that they may be one, I in them and you in me, that they may be brought to perfection as one, that the world may know that you sent me, and that you loved them even as you loved me." (John 17:20-23)
"The bread which we break, is it not a participation in the body of Christ? Because there is one loaf, we who are many are one body, for we all partake of the same loaf." (1 Corinthians 10:16-17)
"I, then, a prisoner for the Lord, urge you to live in a manner worthy of the call you have received, with all humanity and gentleness, with patience, bearing with one another through love, striving to preserve the unity of the spirit through the bond of peace: one body and one Spirit, as you were also called to the one hope of your call; one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all."(Ephesians 4:1-6)
In Pope John Paul II's encyclical Ut Unum Sint ("That They May Be One"), we read:
Second, Catholics can genuinely learn and receive support from other Christians. In fact, Catholics are encouraged to study the beliefs and backgrounds of other Christian churches in order to understand them better and to meet individually or in groups to pray with other Christians. (UR, no. 4)
Third, in discussing our beliefs with other Christians, Catholics should state the teachings of the Catholic Church clearly and non-defensively. As the Decree on Ecumenism instructs, Catholics should avoid a "false conciliator approach" in presenting Catholic belief "which harms the purity of Catholic doctrine and obscures its assured genuine meaning". This decree also encourages Catholics to explain their beliefs "profoundly and precisely, in ways and in terminology which our separated brethren too can really understand." (UR, no. 11)
In addition, we must seek forgiveness and give forgiveness to our non-Catholic Christian brothers and sisters. Separation from the Catholic Church is of the past, and our brothers and sisters in Christ are baptized, believe in the Trinity and in Jesus Christ as Lord and Savior. They have the right to be "honored by the title of Christian, and are properly regarded as brothers and sisters in the Lord by the sons and daughters of the Catholic church." (UR, no. 3)
And finally, Catholics are encouraged to join with all Christians in professing to the whole world our "faith in God, one and three, in the incarnate Son of God, our Redeemer and Lord." Most importantly, our cooperation in matters of social justice is essential. This is possibly the most visible of all the initiatives we can take in our journey of ecumenical outreach, and the one which offers the most potential for a strong, solid cooperative foundation between the Catholic church and the Protestant churches. (UR, no. 12)
For more on ecumenism, see "Basics of the Faith: A Catholic Catechism" by Alan Schreck.