For I am the least of the apostles and do not even deserve to be called an apostle, because I persecuted the church of God. But by the grace of God I am what I am, and his grace to me was not without effect. No, I worked harder than all of them—yet not I, but the grace of God that was with me.  1 Corinthians 15:9-10

            An apostle means someone called to follow Jesus and be his witness.  Many disciples followed Jesus on their own volition.  But apostles were specifically called to follow by Jesus himself.  Being chosen puts you in a special status and hierarchy.