The gospel of Christ required men to speak, to live, and to believe in the message and preaching of the good news. It did not depend on Jesus Christ only for he was above, resting and seated at the right hand of the Father.
When you look at any venture, a mission, a task, a team, a legion, you will know that it depends on very few to bring together the masses to accomplish any goal.
The Apostles were 12 and became the pillars of the universe accomplishing the greatest and most arduous task known to man and to angels and to all creation. Imagine the power necessary to uphold the cosmos, encased and infused in the power of their tongues, to be the rocks upon which the church was built, this being why each are thrones in heaven.