Opening his mouth, Peter said: “I most certainly understand now that God is not one to show partiality, but in every nation the man who fears Him and does what is right is welcome to Him. Acts 10:34-35
Imagine that as Peter was specifically sent to Cornelius is how every missionary is sent, the fruits would be superabundant. Much of effort without fruit today is attributable to operating independence of the Spirit.

Pray that missionaries will yield to the Spirit totally for every assignment.
Receive grace for believers alike to do nothing of their own but by the Spirit.
Pray that none will resist the leading of the Spirit.