That phrase is from Pope Benedict XVI's letter to Ecumenical Patriarch Bartholomew I, dated November 23, 2007.
I have encountered some Christians who strongly oppose efforts toward Christian unity. They view such efforts as intrinsically and necessarily requiring sinful compromise. Their opposition, I think, may be due in large part to a limited imagination, for they seem not to have imagined any other way of achieving genuine Christian unity than through sinful compromise. We can hold out to them the possibility of a holy, passionate, unrelenting effort to achieve genuine unity with other Christians that does not require any sinful compromise. We can help them perceive the possibility that in the course of dialogue resulting from ecumenical effort of this holy type, all participating persons may come to see changes they can make in themselves that are not sinful, but are in fact obedient to Christ.
Other Christians treat ecumenical efforts as worthless or unimportant. These persons tend to think either that unity is already present and perfect, because they think Christian unity is invisible and entirely effected and perfected merely by belief in Jesus, or that true Christian unity is something that will be achieved only in heaven, and that therefore it is foolish to try to bring about Christian unity in this life. Those who think Christian unity is already present and perfect have adopted a view of Christian unity that is far short of the unity Christ prays in John 17 that His followers would have. It is a gnostic conception of unity from which all material unity has been stripped away, the barest of formal unity left remaining. No family with only that sort of unity would be thought of as unified, for it is entirely compatible with bitterness, division, separation, and fragmentation. Christians holding this notion of unity need to be shown that the unity Christ seeks for His followers is far greater than mere "belief in Jesus".
Those Christians who think that there is no point striving for Christian unity in this life should be shown that we are called by Christ to pursue unity in this present life with all our brothers and sisters in Christ, to seek to heal present schisms, and to be reconciled with our separated brothers and sisters, so that by our unity and mutual love for each other all the world will see the love of God the Father for Christ Jesus. Love cannot be perfected when we do not seek unity, for it is the very nature of love to seek unity. Therefore he who does not seek genuine unity with his separated brothers and sisters does not love them as he ought. That is the ecumenical equivalent of saying to the poor, "Go in peace, be warmed and be filled". (James 2:16) Let us say instead to our separated brothers and sisters, "I come in peace; I commit myself to pursuing peace and unity together with you, for this is the will of Christ our Lord."
"Let unity, the greatest good of all goods, be your preoccupation." - St. Ignatius of Antioch (Letter to St. Polycarp)