The apostles equip the church with revelation of the doctrine and the meaning of the Scripture:

Ephesians 1:8-9 "Wherein he [Christ] hath abounded toward us in all wisdom and prudence; Having made known to us [the apostles] the mystery of his [Christ's] will, according to his [Christ's] good pleasure which he [Christ] hath purposed in himself [Christ]:"

If it were not a mystery, that would be a different story.  It is clearly a mystery.  The will of Jesus for the Church is a mystery and He reveals it to apostles who must be faithful in bringing it to the entire church through local elders.

Let God be true and every man a liar.

Apostles are given the responsibility of protecting the entire flock of God from the attack of the enemy, but with so many voices in the world, each one claiming to be the one that should be listened to, how can you know? You are responsible to be close enough to God so that you can tell the difference between false apostles and real apostles.

Read the complete definition of "Apostle."