Kongo did not become Christian because missionaries overcame it. The court adopted the religion deliberately in the 1490s and Afonso made it the framework of royal authority, and for the next two centuries Kongo was a Christian kingdom with its own clergy, its own confraternities and its own saints' days, at a time when Portuguese presence on the ground was thin and often absent for years. What made that possible was that the new vocabulary mapped onto an existing one. Kongo religion already held that the living and the dead occupy two worlds divided by water, that the dead retain power over the living, and that certain objects and specialists mediate between them.
The word chosen for the Christian God, Nzambi a Mpungu, was already in use. The word for a priest, nganga, was the word for a ritual specialist. Holy water, relics and the crucifix entered a category the culture already had for empowered objects. Wyatt MacGaffey argued that what happened is best described as a translation in which both sides believed they were being understood, and each was partly wrong. That analysis is now standard, and it is also what makes the Antonian movement of Kimpa Vita two centuries later intelligible rather than aberrant.