40 Truly we are aware of the accumulation of prejudice that tenaciously prevents the happy fulfillment of the prayer offered by Christ at the last Supper to his Eternal Father for the followers of the Gospel: 'That they may be one' ( John xvii, 21). But we know also that such is the strength of prayer, when those who pray are joined together in a common fervor, a strong faith, and a clear conscience, that it can lift up a mountain and cast it headlong into the sea (cf. Mark ii, 23). We desire then and we wish that all those who have at heart an earnest invitation to Christian unity - and surely no one who belongs to Christ would belittle the importance of this matter - should pour forth their united prayers and supplications to God, from whom comes all unity, order and beauty, that the praiseworthy desires of every right-thinking person may soon be brought to fulfillment. Let research be made without jealousy or anger to straighten out the path by which this good may be reached; let us bear in mind that today we are accustomed to retrace and weigh the events of bygone ages more calmly than in the past.
Source: Sempiternus Rex Christus (Vatican.va)