How did Christianity spread during the Middle Ages?

By the sword.

Clovis's conversion was significant because he was aligning himself with the political forces that the church could muster. As a Christian King, fighting for Christendom, he had the backing and the blessings of the church who benefited from the relationship just the same as he did.

Clovis Middle Ages

Notice how someone can say whatever they darn well please about Catholicism and it isn't questioned as long as it is negative. Anyway. There were many missionaries to places in Europe still unconverted by that time. The lower class loved Christianity for several reasons. One being that the Church gave so many days of rest. In the late Middle ages, some dioceses prescribed almost a third of a year for rest. In additions, remaining pagan rulers had some non-spiritual incentive to convert because of the support they would get from the rich and powerful Christian rulers. Christian clergy were also desirable because of their education, they served as valued councilors to many in power.

For the most part, it didn't. The common people were not allowed to possess a Bible. They were kept in total ignorance spiritually. What was taught didn't come from God's Word, but, from men. At that time, religion had so much power, it controlled every aspect of life.

The same way it attempts to thrive now. Through fear, political pressure, and financial pressure.

Although in the middle ages they also incorporated torture and killing which I hope is never again allowed.

The same way the black plaque did with similar results. It's infectious

Slowly.

Depends on whether you're talking about Catholicism or Protestant religions.

BONUS for me (:

The sword and gold matey

Well, they used oppression, and I believe their preists were very charismatic as well.

No one expects the Spanish Inquisition!!!