European missionaries had occasionally visited Vietnam for short periods of time, with little impact, beginning in the early sixteenth century.

The best known of the early missionaries is Alexandre de Rhodes, a French Jesuit who is sent to Hanoi in 1627, where he quickly learns the language and begins preaching in Vietnamese.

Initially, Rhodes is well-received by the Trinh court, and he reportedly baptizes more than six thousand converts; however, his success probably leads to his expulsion in 1630.

He is credited with perfecting a romanized system of writing the Vietnamese language (quoc ngu), which is probably developed as the joint effort of several missionaries, including Rhodes.

He writes the first catechism in Vietnamese and publishes a Vietnamese-Latin-Portuguese dictionary; these works are the first books printed in quoc ngu.

Romanized Vietnamese, or quoc ngu, is used initially only by missionaries; classical Chinese, or chu nom, continues to be used by the court and the bureaucracy.

The French later support the use of quoc ngu, which, because of its simplicity, leads to a high degree of literacy and a flourishing of Vietnamese literature.

After being expelled from Vietnam, Rhodes spends the next thirty years seeking support for his missionary work from the Vatican and the French Roman Catholic hierarchy as well as making several more trips to Vietnam.