Vatican II: Lumen Gentium
What specifically characterizes the laity is their secular nature. It is true that those in holy orders can at times be engaged in secular activities, and even have a secular profession. But they are by reason of their particular vocation especially and professedly ordained to the sacred ministry. Similarly, by their state in life, religious give splendid and striking testimony that the world cannot be transformed and offered to God without the spirit of the beatitudes. But the laity, by their very vocation, seek the kingdom of God by engaging in temporal affairs and by ordering them according to the plan of God.
They live in the world, that is, in each and in all of the secular professions and occupations. They live in the ordinary circumstances of family and social life, from which the very web of their existence is woven. They are called there by God that by exercising their proper function and led by the spirit of the Gospel they may work for the sanctification of the world from within as a leaven.
In this way they may make Christ known to others, especially by the testimony of a life resplendent in faith, hope and charity. Therefore, since they are tightly bound up in all types of temporal affairs it is their special task to order and to throw light upon these affairs in such a way that they may come into being and then continually increase according to Christ to the praise of the Creator and the Redeemer.
So it is that that messianic people, although it does not actually include all men, and at times may look like a small flock, is nonetheless a lasting and sure seed of unity, hope and salvation for the whole human race. Established by Christ as a communion of life, charity and truth, it is also used by Him as an instrument for the redemption of all, and is sent forth into the whole world as the light of the world and the salt of the earth.
Vatican II: Decree on the Apostolate of the Laity
The laity fulfill this mission of the Church in the world especially by conforming their lives to their faith so that they become the light of the world as well as by practicing honesty in all their dealings so that they attract all to the love of the true and the good and finally to the Church and to Christ.
They fulfill their mission also by fraternal charity which presses them to share in the living conditions, labors, sorrows, and aspirations of their brethren with the result that the hearts of all about them are quietly prepared for the workings of saving grace.
Another requisite for the accomplishment of their task is a full consciousness of their role in building up society whereby they strive to perform their domestic, social, and professional duties with such Christian generosity that their manner of acting should gradually penetrate the whole world of life and labor.