"(a) The distinction between clergy and laity is of divine origin, for first, Christ chose the twelve apostles from among his disciples; and in a special way deputed and consecrated them for the exercise of spiritual ministrations; and
second, the apostles, who could not mistake the will of Christ, administered the sacrament of Holy Orders by consecrating bishops and by ordaining priests and deacons." (. . .)*
Baltimore Catechism No. 3, The Text of the Official Revised Edition 1949, by Rev. Francis J. Connell, C.SS.R., S.T.D.
*(abridged)
"That a man may receive Holy Orders worthily it is necessary:
First, that he be in a state of grace and be of excellent character.
Second, that he have the prescribed age and learning;
Third, that he have the intention of devoting his life to the sacred ministry;
Fourth, that he be called to Holy Orders by his bishop.
a) Some preliminary signs of a vocation to the priesthood are:
First, that the boy or young man be capable of living habitually in the state of grace;
Second, that he be attracted to the priesthood and manifest this attraction by frequent confession and Communion, by a virtuous life, and by a love of serving Mass.
b) Those who are called by God to be priests ordinarily receive no special revelation to this effect. God expects all to use the gifts of reason and of grace in determining their state in life.
c) Without a special dispensation no one may be ordained a priest until he is twenty-four years of age. Ordinarily the prescribed learning consists of four years of high school, four years of college, and four years of theology completed in a seminary.
d) The sacred ministry of the priesthood can be exercised either as a diocesan priest under a bishop, or as a member of a religious community under a religious superior. Priests of religious orders make the vows of poverty, chastity, and obedience. Diocesan priests bind themselves to chastity for life and make a solemn promise of obedience to their bishop."
Baltimore Catechism No. 3, The Text of the Official Revised Edition 1949, by Rev. Francis J. Connell, C.SS.R., S.T.D.
*(abridged)