We are going to cover three areas what it means to be a Christian as common person or is a more fancier the "laity".
- Action in the secular world
Apostleship can refer to different meanings such as an holding a position but for most part refers to “one sent”. What are we sent to do? According to “Apostolicam Actuositatem” from Second Vatican Council, its the duty of teaching, sanctifying and mission. Of course the successors of the Apostles which are the bishops and through his ordained priests and other clergy do have a primary role in this but laity are not excluded.
Before we can “be sent” we need to have that relationship and understanding of who are being sent from and that is discipleship or first being a disciple. A disciple is a key mandate from Jesus. The Greek word for disciple is “mathetes” which came from “manthanein” meaning to learn. The English word comes from latin word “discipulus” which is defined that learning is a process. So with both of these meanings combined and if we are true followers of Christ, we are committed to a lifelong process of learning more about God.
Once we are committed to this process of learning more, it is then we can serve our Lord Jesus Kingdom which is the Holy Catholic Church; that is to be sent and go out to share what we continue to learn more about God. Keep in mind we are not just talking about theology or brainy stuff, we are talking about also relationship, as stated from Second Vatican:
“Since Christ, sent by the Father, is the source and origin of the whole apostolate of the Church, the success of the lay apostolate depends upon the laity's living union with Christ, in keeping with the Lord's words, "He who abides in me, and I in him, bears much fruit, for without me you can do nothing" (John 15:5). This life of intimate union with Christ in the Church is nourished by spiritual aids which are common to all the faithful, especially active participation in the sacred liturgy.(5) These are to be used by the laity in such a way that while correctly fulfilling their secular duties in the ordinary conditions of life, they do not separate union with Christ from their life but rather performing their work according to God's will they grow in that union. In this way the laity must make progress in holiness in a happy and ready spirit, trying prudently and patiently to overcome difficulties.(6) Neither family concerns nor other secular affairs should be irrelevant to their spiritual life, in keeping with the words of the Apostle, "What-ever you do in word or work, do all in the name of the Lord Jesus Christ, giving thanks to God the Father through Him" (Col. 3:17). “
So in other words, “talk the talk...walk the walk”. It is important that our relationship with God is through prayer and keeping up with “faith, hope and charity”. It is then we can truly practice apostleship in a glorified way and for God's will , and not for our glory or our will. So through the church, there are many apostleship activities and Legion of Mary is one of these ways where we go out to evangelize and conservation of saving souls through door to door, bible studies, leading in prayer, faith formation, and youth groups, etc.
Vocation also can mean different things but basically it's utilizing our gifts or talents that is given to each of us so to lead us to a specific purpose in life. That purpose links to the two greatest commandments Jesus reminded and taught us “Love our God with all my heart, soul and mind and love my neighbor as myself”. So in following these commandments we strive for holiness and mission to others in this world.
The Second Vatican also states: For the Christian vocation by its very nature is also a vocation to the apostolate. No part of the structure of a living body is merely passive but has a share in the functions as well as life of the body: so, too, in the body of Christ, which is the Church, "the whole body . . . in keeping with the proper activity of each part, derives its increase from its own internal development" (Eph. 4:16).
So for us what does this means. As St. Paul noted in 1 Corithians 12:5-7”There are different kinds of spiritual gifts but the same Spirit;there are different forms of service but the same Lord;there are different workings but the same God who produces all of them in everyone. To each individual the manifestation of the Spirit is given for some benefit. “ .
So vocations can be different for everyone and also be layered but the same goal, love and serve God. For example a person can follow the vocation of marriage by keeping the marriage bond and within that marriage their vocation is having a large family, raise children as disciples of God and good citizens as a “domestic church” or their could be a married couple unable to have children but utilize their love and service to the church and others with more charity as their vocation. Same is with a single person deciding to be a priest or nun but within their vocation of celibacy they can serve a parish, serve the poor, teach or be a missionary as their sub vocation. All act differently in some way but have the same nature, similar to how the Trinity is.
Action in the secular is fairly simple, it's just simply “doing” or “fulfilling” what was noted earlier in terms of apostleship and vocation. However, it's doing it in a environment that is outside our box. It's one thing and fulfilling your apostleship and vocation with a parish, it's another to do say in your work place, school, grocery store, and other secular public events. It's a challenge and takes grace, which is why we can't do it alone and why we need to go to mass weekly on Sunday, where we are feed with the Word of God and the Eucharist, and then follow the priest or deacons words end of the mass “go forth or go and announce the Gospel of the Lord”.
Actually all of these points are documented in detail and fairly clear to understand from Second Vatican Council document, just google “Apostolicam Actuositatem” which is “Decree of Apostolate of the Laity” . Keep in mind Frank Duff was one of the few laity to be an observer in Second Vatican Council.