So, an Apostle must be filled with the Holy Spirit have supernatural spiritual revelation of the Word and supernatural discernment of truth and lie.   

When you look at Paul`s journey to spiritual maturity (his revelation of the Gospel did not bring him maturity), he began by making a case that he was an Apostle (pride?).   Many of his epistles start out with this affirmation.   

However, as he progressed in his relationship with Christ he began to refer to himself the as servant of Jesus Christ and said that he was the "least of the apostles".   

I believe one reason that Paul declared himself an apostle so vociferously in his writings was to get the attention of the church in order to deliver the full revelation of the Gospel which he received directly from the Holy Spirit so that it would be accepted by those to whom it was revolutionary.   

If a person takes on the title of Apostle they put themselves on the judgment seat and will be held accountable for speaking as the oracle of God.