Like Amos in our first reading, "the Twelve," as the inspired author of Mark's Gospel calls Jesus's closest disciples, are not professional prophets or teachers. Also like Amos, it is precisely their lack of credentials that make it necessary for them to rely solely on God to accomplish what they were sent them to do. In Amos's case, he was sent to prophesy at the shrine of Bethel in the northern kingdom, what was usually referred as "Israel" (as opposed to the southern kingdom of Judah, where Jerusalem was located).
It is important to point out that Bethel was the major religious center of the northern kingdom. Hence, the invitation extended to him to go prophesy in Judah and not to continue his prophesying at Bethel. At that time there were apparently schools of professional prophets whose "job" it was to prophesy. Often this amounted to just saying things the leaders and people wanted to hear. Amos was intent on bringing them the message God wanted to them hear, which was one to which they were not terribly receptive. This explains Amos's defensive retort that he was a shepherd and "dresser" of sycamore trees," not a prophet. His reason for speaking out was not personal gain but because God told him to speak. The message of his prophesying was to call Israel (i.e., the northern kingdom) back to fidelity with God by adherence to the covenant.
On the other hand, Jesus sent the Twelve to preach repentance, cast out demons, and heal the sick. In other words, the Lord sent them to carry out and to perhaps further his own mission. In so doing, they were to rely solely on God by taking nothing with them except the clothes on their backs (no extra clothes) and confidence that God would provide them on their mission.
At root, Amos's message was to point out how badly Israel had betrayed their God and broken the covenant by their lack of care for widows and orphans. He also lambasted them for cheating the less well-off when trading and other like misbehaviors. According to the author of the Introduction to the Book of Amos found on the NABRE on-line version of the Bible, Amos insisted that religious observance "without justice is an affront to the God of Israel and, far from appeasing God, can only provoke divine wrath."
When preaching repentance, the Twelve were to no doubt echo (the word "catechesis," which is Greek in origin, means to "echo" or "resound") Jesus's own preaching, which is summarized in Mark's Gospel thus: "The kingdom of God is at hand. Repent, and believe in the gospel" (see Mark 1:14-15).
Like the Twelve, who had already left everything to heed Jesus's call (see Mark 1:16-20), Amos's embarking on his prophetic mission likely came at great cost to himself. It is no small thing to leave your flock and field behind in order to go to the major religious shrine of your nation and call on the political and religious leaders to repent. In an oracle directed at the northern kingdom, Amos famously inveighed:
For three crimes of Israel, and now four— I will not take it back [his rejection of them]— Because they hand over the just for silver, and the poor for a pair of sandals; They trample the heads of the destitute into the dust of the earth, and force the lowly out of the way. Son and father sleep with the same girl, profaning my holy name. Upon garments taken in pledge they recline beside any altar [worship of false gods by the practice of usury]. Wine at treasury expense they drink in their temples (Amos 2:6-8)
For whatever reason, culturally, we are inclined to zero in on the sexual immorality piece. But we do so from a moral perspective that was unknown to Amos and his hearers. The Hebrew verb (which transliterates into English as ilku) translated in the NABRE, from which I took the citation, as "sleep with," is a bit ambiguous. Literally, it means something like "go in unto." Most scholars agree that it probably refers to a man and his son having sexual relations with the same young woman. Because what Israel is being lambasted for in this oracle has to with injustice and oppression, this verse "perhaps suggests [sexual] exploitation" (Jennifer M. Dines, "Amos" in The Oxford Bible Commentary, 583). In other words, it likely refers to prostitution and/or the exploitation of poor young women for whom it is necessary to go into servitude to eek out a living. That such exploitation took place is witnessed by the fact that in the Book of Ruth Boaz sought to protect Ruth from just this kind of thing when she turned up as a gleaner is his part of the communal field: "Listen, my daughter. Do not go to glean in anyone else’s field; you are not to leave here. Stay here with my young women. Watch to see which field is to be harvested, and follow them. Have I not commanded the young men to do you no harm?" (see Ruth 2).
Based on Amos's response to God's call and the response of the Twelve to Jesus, you'd think it was the most important thing in the world. Well, for one who has heeded it, God's call is the most important thing in the world. God will never make you do what he calls you to do. He calls and allows the one he calls the freedom to respond or not. You were called by name when you were baptized. Your call is to participate in Christ's prophetic, priestly, and royal mission. You were sealed and further strengthened for this call when you anointed in confirmation. This call is renewed and you are strengthened to undertake it each Sunday at Mass, at the end of which you are sent (i.e., dismissed) to carry it out.
While it also refers to a specific office in the Church (one that only twelve, perhaps 13- if you count Matthias, who was chosen to replace Judas - people were ever called to; the office continues because bishops are "successors of the apostles" and exercise the apostolic office), at root the word "apostle" means "one who is sent for a purpose." When, in the creed, we confess that the Church is "apostolic" (as well as "one, holy, catholic") we refer at one and the same to the apostolic office, which is handed on through apostolic succession, and to our being sent forth with words like, "Go and announce the Gospel of the Lord" (one of four authorized dismissals - the other three being "Go forth, the Mass is ended," "Go in peace, glorifying the Lord by your life," or simply, "Go in peace" - The Roman Missal, sec. 144). Like Amos and the Twelve, you can be sure that heeding this call will cost you something. But, then again, it is the most important thing in the world.