From where did Pope JPII--and other Church theologians--take the notion of "seeds of the Word"? This notion came from the "Patristic times"--from the times of the Church Father. Among the Church Fathers was St. Justin the Martyr who really reflected on "seeds of the Word".
St. Justine was a philosopher who wrote in Greek. He was a convert to Christianity. He was born in Samaria and went to Rome sometime 150 AD. There in Rome he started a school in philosophy marked by Christianity. In 167 he was martyred. This is why the Church calls him "Justin the Martyr".
He said that before the coming of Christ all humanity had access to partial truths, all humanity received "seeds of the Word". Humanity still participated in the divine mind of God. The human mind had imperfect and partial knowledge of truths about Revelation.
When Christ came the summit of Revelation happened. Christ was the ultimate and full Revelation of God. "The Word became flesh" (Jn1/14). Now the truth is full and out in the open. While Christ was not yet received by others, the truths that others knew were partial--just "seeds of the Word".
Let us cite some of what he wrote:
Passage #1: "Our doctrines, then, appear to be greater than all human teaching; because Christ, who appeared for our sake, became the whole rational being.... For whatever either lawgivers or philosophers uttered well, they elaborated by finding and contemplating some part of the Word. But since they I did not know the whole of the Word, which is Christ, they often contradicted themselves" (2Apology 10).
Passage #2: "...I confess that I both boast and with all my strength strive to be found a Christian; not because the teachings of Plato are different from those of Christ, but because they are not in all respects similar, as neither are those of the others, Stoics, and poets, and historians. For each man spoke well in proportion to the share he had of the SPERMATIC WORD, seeing what was related to it. [.... ] ... we worship and love the Word who is from the unbegotten and ineffable God, since also He became man for our sakes, that becoming a partaker of our sufferings, He might also bring us healing. For all the writers were able to see realities darkly through the SOWING of the implanted word that was in them. For the SEED and imitation impacted according to capacity is one thing, and quite another is the thing itself, of which there is the participation and imitation according to the grace which is from Him" (2Apology 13).
Let us try to explain these passages.
In #1 St. Justin says that Christ is fullness of knowledge. Those who do not know Christ "elaborate" and "contemplate" partially the Word. They do not see and understand completely. They understand partially. Because of partial knowledge, they tend to contradict each other.
In #2 we really see the words used by St. Justine in relationship to "seeds" of the Word. St. Justin is proud that he is Christian. Other persons--like Plato--who do not know Christ have teaching similar to the teachings of Christ. Those who do not know Christ see parts of the truth of Christ--they have a proportion of the "spermatic Word". Those other people see things in a darker way, not fully bright. Why? They have the word implanted in them, somehow. It is already "sowed" in them, somehow. Of course, says St. Justin, have seeds is not as full as the person of Christ himself.
Later, in our era, the Vatican II council picked up this notion of "seeds of the Word" taking mainly from St. Justin (and Clement of Alexandria). The document Gaudium et spes (#3) mentions that the council proclaims the destiny of humanity and "champions the Godlike seed" that has been sown in humanity. Another council document, Ad gentes, mentions this too. Other people belonging to other religious traditions "may be able to bear more fruitful witness to Christ" so "let them gladly and reverently lay bare the seeds of the Word which lie hidden among their fellows". (Ad gentes #11). So the Vatican II council acknowledge that others--even if they are not Christians--can bear witness to Christ because the "seeds"have been sown in them.
Now as we read Pope John Paul II's Redemptoris missio we can have an idea of where the notion "seeds of the Word" come.