16 Now what We have touched upon above, must be particularly borne in mind, namely, that what still remains to be accomplished in this field calls for an enormous effort and innumerable laborers. Let us remember that our brethren "who sat in darkness and shadow" form an immense multitude that can be reckoned at about 1,000,000,000. Hence it appears that the ineffable sigh of the most loving Heart of Christ is echoing still: "And other sheep I have that are not of this fold: them also I must bring. And they shall hear my voice: and there shall be one fold and one shepherd."
Source: Evangelii Praecones (Vatican.va)