69 What we have written thus far applies primarily to those nations where the Catholic religion is already firmly established. In mission lands it will not be possible to accomplish all these things until the number of Christians has grown sufficiently, larger church buildings have been erected, the children of Christians properly attend schools established by the Church and, finally, until there is an adequate number of sacred ministers. Still We urgently exhort apostolic workers who are laboring strenuously in these extensive parts of the Lord's vineyard to pay careful attention to this matter as one of the serious problems of their ministry.
Source: Musicae Sacrae (Vatican.va)