His cleansing of the seat of power extended to the Portuguese, cutting ties with them. He saw their intrusion into the affairs of the kingdom as a threat to his rule. In 1555, he deported all 70 Portuguese nationals from his kingdom. This is documented in a legal inquest in 1550 into an attempt to oust him by the former king, Pedro Nkanga a Mvemba, according to Dbpedia.
Historical accounts indicate that the former king hatched his plot from the church where he was seeking refuge. The conspiracy was planned with the support of some Kongolese lords who pledged allegiance to him.
Diogo implored his magistrate and purveyor Jorge Afonso to carry out the inquest. He took testimonies from Kongolese lords who had sworn an oath to Diogo and the plotters, as well as those who claimed to be neutral in the kingdom.
The king had a bigger task of purging the kingdom of conspirators, many of whom were in his court and were loyal to the former king. He risked making a dozen enemies if he swiftly cut off the plotters. On the other hand, any delay meant the deepening of the threat to his rule.
With a conscious but deliberate move, he replaced those loyal to the former king in phases and quickly so he did not get cold feet in expelling them. When he silenced his detractors, Diogo turned his attention to limiting the influence of the Portuguese on the internal affairs of Kongo.
According to history, he had problems with the Portuguese settlers at Sao Tome who were called Tomistas. Per an agreement between Kongo and Portugal, the Portuguese were only to trade within Kongo’s region for slaves. In other words, the Portuguese were to take slaves given by Diogo or people he made to sell slaves. But the Portuguese did not go according to the agreement.
Just World News writes that “the factor that actually broke the deal was the Tomista habit of sailing upriver to the Malebo Pool to purchase slaves from BaTeke traders who were increasingly taken with European goods over the nzimbu shells the manikongo offered them. Enraged by this breach of contract, King Diogo broke off relations in 1555.”
Ahead of this, Diogo had brought in another faith to rival the Catholic Church. Catholic Church had arrived in the Kingdom of Kongo shortly after the first Portuguese explorers reached its shores in 1483. But in 1548 during his reign, Diogo made space for Jesuit Catholic missionaries. It was rumored that this was due to opposition from secular and Capuchin priests. Diogo in 1555 switched loyalty from the Jesuits, prompting him to deport them from the region after they had asked him to surrender everything he owned.
In 1557, he brought Franciscan missionaries to the kingdom and ascribed to their faith. He stayed loyal to them until his death in 1561.