Objection 1. It would seem that there should not be different duties or states in the Church. For distinction is opposed to unity. Now the faithful of Christ are called to unity according to John 17:21-22: "That they... may be one in Us... as We also are one." Therefore there should not be a distinction of duties and states in the Church.
Objection 2. Further, nature does not employ many means where one suffices. But the working of grace is much more orderly than the working of nature. Therefore it were more fitting for things pertaining to the operations of grace to be administered by the same persons, so that there would not be a distinction of duties and states in the Church.
Objection 3. Further, the good of the Church seemingly consists chiefly in peace, according to Psalm 147:3, "Who hath placed peace in thy borders," and 2 Corinthians 13:11, "Have peace, and the God of peace... shall be with you." Now distinction is a hindrance to peace, for peace would seem to result from likeness, according to (Sirach 13:19), "Every beast loveth its like," while the Philosopher says (Polit. vii, 5) that "a little difference causes dissension in a state." Therefore it would seem that there ought not to be a distinction of states and duties in the Church.
On the contrary, It is written in praise of the Church (Psalm 44:10) that she is "surrounded with variety": and a gloss on these words says that "the Queen," namely the Church, "is bedecked with the teaching of the apostles, the confession of martyrs, the purity of virgins, the sorrowings of penitents."
I answer that, The difference of states and duties in the Church regards three things. On the first place it regards the perfection of the Church. For even as in the order of natural things, perfection, which in God is simple and uniform, is not to be found in the created universe except in a multiform and manifold manner, so too, the fulness of grace, which is centered in Christ as head, flows forth to His members in various ways, for the perfecting of the body of the Church. This is the meaning of the Apostle's words (Ephesians 4:11-12): "He gave some apostles, and some prophets, and other some evangelists, and other some pastors and doctors for the perfecting of the saints." Secondly, it regards the need of those actions which are necessary in the Church. For a diversity of actions requires a diversity of men appointed to them, in order that all things may be accomplished without delay or confusion; and this is indicated by the Apostle (Romans 12:4-5), "As in one body we have many members, but all the members have not the same office, so we being many are one body in Christ." Thirdly, this belongs to the dignity and beauty of the Church, which consist in a certain order; wherefore it is written (1 Kings 10:4-5) that "when the queen of Saba saw all the wisdom of Solomon... and the apartments of his servants, and the order of his ministers... she had no longer any spirit in her." Hence the Apostle says (2 Timothy 2:20) that "in a great house there are not only vessels of gold and silver, but also of wood and of earth."
Reply to Objection 1. The distinction of states and duties is not an obstacle to the unity of the Church, for this results from the unity of faith, charity, and mutual service, according to the saying of the Apostle (Ephesians 4:16): "From whom the whole body being compacted," namely by faith, "and fitly joined together," namely by charity, "by what every joint supplieth," namely by one man serving another.
Reply to Objection 2. Just as nature does not employ many means where one suffices, so neither does it confine itself to one where many are required, according to the saying of the Apostle (1 Corinthians 12:17), "If the whole body were the eye, where would be the hearing?" Hence there was need in the Church, which is Christ's body, for the members to be differentiated by various duties, states, and grades.
Reply to Objection 3. Just as in the natural body the various members are held together in unity by the power of the quickening spirit, and are dissociated from one another as soon as that spirit departs, so too in the Church's body the peace of the various members is preserved by the power of the Holy Spirit, Who quickens the body of the Church, as stated in John 6:64. Hence the Apostle says (Ephesians 4:3): "Careful to keep the unity of the Spirit in the bond of peace." Now a man departs from this unity of spirit when he seeks his own; just as in an earthly kingdom peace ceases when the citizens seek each man his own. Besides, the peace both of mind and of an earthly commonwealth is the better preserved by a distinction of duties and states, since thereby the greater number have a share in public actions. Wherefore the Apostle says (1 Corinthians 12:2)4,25) that "God hath tempered [the body] together that there might be no schism in the body, but the members might be mutually careful one for another."
Source: Summa Theologica (New Advent)