To understand the apostles' doctrine, even when the apostles are right there teaching it to us, we need the Holy Spirit of revelation. 

Ephesians 1:17-18 That the God of our Lord Jesus Christ, the Father of glory, may give to you the spirit of wisdom and revelation [Apokalupsis: removal of a covering or veil] in the knowledge of him:  The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints

Apocalypse (two Greek words, apokalupsis, and apokalupto meaning approximately the same thing) Literally, it means the removal of a covering or a veil. God often provides the interpretation of Scripture within Scripture. There is one Scripture that gives us the key to the word, veil. Hebrew 10:19-20 says, Having therefore, brethren, boldness to enter into the holiest by the blood of Jesus, By a new and living way, which he hath consecrated for us, through the veil, that is to say, his flesh;

Revelation is always the unveiling of Christ.  God pulls back the veil.  We, with unveiled faces, have our eyes opened to the revelation of Jesus.  This is the way we could understand and be born again.

It is vital that we know Jesus.  This is not the same as knowing about Jesus.  We are literally learning Christ.

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."