How did Clovis and Charlemagne contribute to spread of Christianity?

Clovis' principle contribution was marrying Clotilde who was Catholic and as a result forcing all his subjects to become Roman Catholic--they were Arians before that. In fact the vast majority of Christians were Arians until this time, but this conversion threw the majority over to Rome.

Wikipedia says: "Clovis was converted to Catholicism;[6] at the instigation of his wife, Clotilde, a Burgundian Gothic princess who was a Catholic in spite of the Arianism which surrounded her at court. The followers of Catholicism believe that God, Jesus and the Holy Spirit are three persons in one God, as opposed to the Arian Christianity, whose followers believed that God and Jesus were two completely separate beings and the Holy Spirit was not entirely separate from God but was the Power of God, a belief system common among the Goths who ruled most of Gaul at the time."

Charlemagne's primary contribution to Christianity was to force the Saxons at sword-point to become Christians.