There is a good quote from the CDF on this:
“Although non-Christians can be saved through the grace which God bestows in “ways known to him,” the Church cannot fail to recognize that such persons are lacking a tremendous benefit in this world: to know the true face of God and the friendship of Jesus Christ, God-with-us. Indeed “there is nothing more beautiful than to be surprised by the Gospel, by the encounter with Christ. There is nothing more beautiful than to know him and to speak to others of our friendship with him.” The revelation of the fundamental truths about God, about the human person and the world, is a great good for every human person, while living in darkness without the truths about ultimate questions is an evil and is often at the root of suffering and slavery which can at times be grievous. This is why Saint Paul does not hesitate to describe conversion to the Christian faith as liberation “from the power of darkness” and entrance into “the kingdom of his beloved Son in whom we have redemption and the forgiveness of our sins.” (Doctrinal Note on some Aspects of Evangelization, CDF)
This makes it clear that non-Catholics, and those who do not know the face of Christ explicitly, even if they are united to him implicitly, nevertheless lack a real good. To make an analogy, one could consider a father who has a son and a servant working for him. The son works together with the father, helping him with his job; he is close friends with him. The servant works in a dusty office, and has a long and boring job; he never sees the father. Both the son and the servant are paid. Does this mean that there is no advantage for the son over the servant?
We can see this in the above quoted text from the CDF: "Such persons are lacking a tremendous benefit in this world: to know the true face of God and the friendship of Jesus Christ, God-with-us." What a tremendous loss, to not be able to know explicitly the God whom one is serving. In a certain sense, this puts a limit on the degree one can love God.
Someone who does not know who Jesus Christ is, that he became man out of love for men, and died a terrible death for men, will have a much harder time eliciting the same act of love that a man who does know this. It is easier for one to love Christ when one sees how much Christ loves him. Thus, because Christ's act was such a great manifestation of his love for man, one who knows this act will be able to love God more. The following quotation from the Catechism supports this point:
"The Word became flesh so that thus we might know God's love: "In this the love of God was made manifest among us, that God sent his only Son into the world, so that we might live through him." "For God so loved the world that he gave his only Son, that whoever believes in him should not perish but have eternal life." (CCC 458)As the CDF pointed out, St. Paul calls acceptance of the Gospel a liberation "from the power of darkness," and an entrance into "the kingdom of his beloved Son in whom we have redemption and the forgiveness of our sins." This goes along with what I have been saying, and is why the Church is so strongly urging missionary activity in the Church, even now. Missionary activity is necessary to bring the light and goodness of Christ to all peoples, so that they might become friends of Christ, and be saved in Him.
It should be clear how a statement such as this is compatible with saying that there can be some among those ignorant of Christ who are seeking God with a sincere heart, and are justified by their implicit faith. This does not detract from the importance of the Gospel; the justification of such a man only comes through Christ, regardless of whether he knows it or not. Christ is what such a man is seeking, and the Church has a grave obligation to bring Christ to all peoples.