What is the difference between being a disciple and an apostle? Except for Judas, Iscariot, what happened that the remaining eleven should have their descriptor changed?
The definitional difference in these words is easy. A disciple is a student, someone being taught, formed, or disciplined into a way of living, learning, or being. An apostle is one who is sent. The term is originally Greek and was applied to inanimate things like a bill or an invoice. In the New testament it is applied to the persons sent to proclaim the Good News of Jesus Christ.
Broadly speaking, in the Gospels, the Twelve are normally referred to as disciples, although when they are introduced in their lists in the several Gospels, they are called apostles. That may be because by the time the story was told, they were acting as apostles.
It is interesting that in today’s readings from Acts and John, both of which Gospel writers are telling the readers about how the twelve are empowered by the Holy Spirit to become those who are sent to proclaim what God did in Jesus, i.e. Apostles. Now in neither reading is the term Apostle used. In Acts 1.24-25, just before the coming of the Holy Spirit, a new man, Matthias is added to the apostleship of the twelve. Therefore, that term is now implied for the twelve following this moment in their ministry. In John’s reading, the eleven are still referred to as disciples, as they always are in John. It is not until Revelation that Johannine literature refers to apostles at all.
So, the short answer is that disciples are persons in formation and apostles are those who have been sent to witness.
On that topic, the following is a quick reflection on apostleship in a period of sheltering in place and the coming regathering of the community with limits. What are faithful apostles to do? The answers seem to be embedded in the story just before the Pentecost reading in Acts, specifically Acts 1:12-14. When the eleven return from witnessing and being commissioned at the ascension, they gather to pray. In fact, they devote themselves to prayer. Prayer, both petitioning and listening is a purposefully holy act. It is conversation with God and through his Holy Spirit, God answers. Waiting and listening in prayer is never a wasted time.
There is another point to this reflection. The church can do things while waiting on God’s response—it can serve the least of its community, it can teach, it can worship, it can connect to those outside its walls. St Francis continues all these ministries even while it shelters in place!