Towards the end of Jesus' earthly ministry, He gave us an important assignment, mostly called the Great Commission: to preach the good news to every creature. He mentioned a prerequisite we would need before proceeding to witness to the entire world about Him the power of the Holy Ghost.
Going out on the work of great missions without the power of the Holy Ghost is suicidal. This is why we have victims among present-day missionaries because some of them aren't spiritually equipped. While they have the zeal to do the work of God, many people lack the spiritual enablement for the work at this present moment, just like Uzzah when the Ark of the Covenant was being moved.
Overconfident sin killed Uzzah. This is the reason David prayed that the Lord should set him free from arrogant sins. We need to be empowered by the Holy Ghost before we can be effective missionaries. While the missionary assignment must be kept by true children of God, we must make sure to receive the power of the Holy Spirit before we start.
We must also be baptized in the Holy Spirit and going by the gift which the Holy Ghost, we can see the reason every missionary and soul-winner must be possessed by the Holy Spirit. For instance, the word of wisdom is essential for good evangelism. An evangelist without the Holy Ghost is an ordinary orator.
A missionary needs the gift of the Holy Spirits as a tool against the trick of the enemy. A missionary life without speaking and praying in tongues would be in chaos.