Christian missionaries have long wrestled with the fact that those people they evangelize often have something to teach them as well. Christianity offers Christ. With Christ, the Church is the pillar and ground of the truth. The Church is able to collect and integrate all truth into herself because the Church is the Body of Christ, and Christ is the Logos from which all logoi, even human-made logoi, flow.
It has often been said that the method by which missionaries would engage other cultures and peoples is to take seriously whatever truths and wisdom those cultures possessed, whatever is good and useful which was found in those cultures, and “baptize them,” purifying them through contact with the truths of the Christian faith. Whatever impurities found in them would be removed, and then what they have to offer to the whole of humanity can be affirmed. However, it is also possible to say that in the process of doing so, these newly accepted truths also help Christians purify themselves from their own faulty reasoning, because they offer something Christians had yet to consider but once accepted into the faith, the implications of these new truths will transform the way Christians think and act in the world.
In engaging the world and bringing the truths found into the world together into the Church, Christians certainly gave much in return, for they became vessels of grace which perfected what was found in the world, raising it up to a new, and improve status. The key to missions was to realize how much truth, including religious truths, Christians could find in those communities they evangelized. It could be said the best form of missionary activity was when Christians witnessed and approved of the intuitive religious sensibilities of the cultures they engaged; their work was not to go around destroying all that was non-Christian, but raising it all up through the grace of the Holy Spirit. Instead of putting out the flame of religious piety, they used the embers of truth found throughout the world and blew into it the Spirit of Christ, turning those embers into full-functioning fires, revealing the great light of truth found throughout the world.
It is in this spirit we find missionaries converting ancient pagan sites for Christian use, and upon conversion, teaching priestly figures from those sites so that they could be ordained and fulfill their priestly calling in Christ. We find that such sites, having been transformed into churches, not only continued to serve the same function as they did before they had been put to Christian use, they found their position and calling elevated by the grace of Christ. In this way, we find Christian missions served to affirm the positive religious values of the cultures they engaged, so that the people in them could remain to true to themselves and their heritage while adding to it the complementary truths and grace provided by Christ. Married pagan priests became married Christian priests; holy pagan days found themselves fulfilled in Christian holy days; pagan holy shrines became places of Christian miracles, so that their pagan past was even able to be seen as a prefiguration of their Christian future.Pope St. Gregory the Great, in his letter to Abbot Mellitus, represents one way this was acknowledged by the leaders of the Church. In it, the Pope explained how pagan temples were to be converted to Christian use:
The idols are to be destroyed, but the temples themselves are to be aspersed with holy water, altars set up in them, and relics deposited there. For if these temples are well-built, they must be purified from the worship of demons and dedicated to the service of the true God. In this way, we hope that the people, seeing that their temples are not destroyed, may abandon their error, and flocking more readily to their accustomed resorts, may come to know and adore the true God. And since they have a custom of sacrificing many oxen to demons, let some other solemnity be substituted in its place, such as a day of Dedication or the Festivals of the holy martyrs whose relics are enshrined there. On such occasions they might will construct shelters of boughs for themselves around the churches that were once temples, and celebrate the solemnity with devout feasting.
While much adaptation was done on the fly, with missionaries determining what could and could not be used by Christians, or through elaborate humanistic enterprises, where scholarly consideration was given to the beliefs and practices of a particular nation, comparing and contrasting them to the Christian faith, showing what could be brought over into the faith through such a process (as found, for example in the writings of the early Christian apologists, or later Christian philosophers like Boethius who took pagan wisdom seriously), there was also another way this integration was done, and that was through legendary representation of Christian missions to pagan lands. In them, miracles of the Apostles or famous Christian missionaries often incorporated elements of the religious heritage of the lands they evangelized, revealing the way the cultural heritages of those people pointed to and gave way to the truths of Christ. Often, such legends included some sort of miraculous adaptation of a pagan ritual site that brought about conversion; a prime-example of this is the material surrounding the Apostle Andrew.