It would appear that Christ would just happen to pass by them one day and call them to be his apostles, asking them to “follow me.” And wonder of wonders, they would immediately respond to follow him, even “leaving everything behind” (relictis omnibus).
We obviously would be struck by the seeming randomness with which these brothers were called, especially considering that what they were asked to do demanded everything from them.
I guess the only plausible answer to that question is that Christ had all the right to do so, and the person called also had the duty to respond accordingly, because in the final analysis, all of us are actually meant to be an apostle. That is to say, to be some kind of ambassador, a representative of Christ on earth.
At bottom, the answer is because we are supposed to be like Christ, another Christ, if not Christ himself (“alter Christus,” and even “ipse Christus”). All of us are patterned after Christ, and so we cannot avoid being involved in the mission of Christ which is the salvation of all mankind.
We are all meant to be apostles of Christ with the lifelong concern for doing apostolate, taking advantage of all the occasions and situations in life. Vatican II spells it out very clearly. “The Christian vocation is by its very nature a vocation to the apostolate.” (Apostolicam actuositatem, 2) So, anyone who wants to be truly consistent to his Christian identity and calling should realize ever deeply that he is called to help others get closer to God. This is what apostolate is all about.
This duty actually springs first of all from our nature. We are not only individual persons. We are also a social being. Our sociability is not an optional feature. It is part of our essence, violating which would be equivalent to violating our very own nature.
We can never live alone. We need to be with others. And more, we need to care for one another. We have to be responsible for one another. And while this caring and loving starts with the most immediate material human needs like food, clothing, etc., it has to go all the way to the spiritual and the more important needs of ours.
That’s why we need to practice affection, compassion, understanding, patience and mercy on everyone. We have to understand though that all these can only take place if they spring from and tend towards God, “the source of all good things” for us.
And the best way we can show our love and concern for one another is when we bring them to Christ, when we help them in our common endeavor to be true children of God, seeking sanctity and cooperating in the work of continuing apostolate.
We should be most aware of this important aspect of our Christian identity and do everything to pursue and develop it. We should not hesitate to give everything for this purpose, as we always have been reminded that since we are freely given by God with so many good things, we should also freely give ourselves to him and to everybody else. (cfr. Mt 10,8)