Jesus did say that the harvest was great. And this used to be the predominate view concerning God's plan for his creation.
Open this old book at read the first few chapters. What naturally complements the great harvest is the issue of the harvesters. The point of this book is that every Christian can take part in this eminent victory. Take part as the Kingdoms of this world become the Kingdoms of the Lord and of his Christ.
The work is not just for the 'clergy' or the 'ordained' or the 'called'. It's not that we must - it's that we are allowed and honored to lead others to the Water of Life.