Objection 1. It would seem that there is but one religious order. For there can be no diversity in that which is possessed wholly and perfectly; wherefore there can be only one sovereign good, as stated in I, 6, A2,3,4. Now as Gregory says (Hom. xx in Ezech.), "when a man vows to Almighty God all that he has, all his life, all his knowledge, it is a holocaust," without which there is no religious life. Therefore it would seem that there are not many religious orders but only one.
Objection 2. Further, things which agree in essentials differ only accidentally. Now there is no religious order without the three essential vows of religion, as stated above (186, 6,7). Therefore it would seem that religious orders differ not specifically, but only accidentally.
Objection 3. Further, the state of perfection is competent both to religious and to bishops, as stated above (185, 5,7). Now the episcopate is not diversified specifically, but is one wherever it may be; wherefore Jerome says (Ep. cxlvi ad Evan.): "Wherever a bishop is, whether at Rome, or Gubbio, or Constantinople, or Reggio, he has the same excellence, the same priesthood." Therefore in like manner there is but one religious order.
Objection 4. Further, anything that may lead to confusion should be removed from the Church. Now it would seem that a diversity of religious orders might confuse the Christian people, as stated in the Decretal de Statu Monach. et Canon. Reg. [Cap. Ne Nimia, de Relig. Dom.]. Therefore seemingly there ought not to be different religious orders.
On the contrary, It is written (Psalm 44:10) that it pertains to the adornment of the queen that she is "surrounded with variety."
I answer that, As stated above (186, 7; 187, 2), the religious state is a training school wherein one aims by practice at the perfection of charity. Now there are various works of charity to which a man may devote himself; and there are also various kinds of exercise. Wherefore religious orders may be differentiated in two ways. First, according to the different things to which they may be directed: thus one may be directed to the lodging of pilgrims, another to visiting or ransoming captives. Secondly, there may be various religious orders according to the diversity of practices; thus in one religious order the body is chastised by abstinence in food, in another by the practice of manual labor, scantiness of clothes, or the like.
Since, however, the end imports most in every matter, [Aristotle, Topic. vi 8 religious orders differ more especially according to their various ends than according to their various practices.
Reply to Objection 1. The obligation to devote oneself wholly to God's service is common to every religious order; hence religious do not differ in this respect, as though in one religious order a person retained some one thing of his own, and in another order some other thing. But the difference is in respect of the different things wherein one may serve God, and whereby a man may dispose himself to the service of God.
Reply to Objection 2. The three essential vows of religion pertain to the practice of religion as principles to which all other matters are reduced, as stated above (Question 186, Article 07). But there are various ways of disposing oneself to the observance of each of them. For instance one disposes oneself to observe the vow of continence, by solitude of place, by abstinence, by mutual fellowship, and by many like means. Accordingly it is evident that the community of the essential vows is compatible with diversity of religious life, both on account of the different dispositions and on account of the different ends, as explained above.
Reply to Objection 3. In matters relating to perfection, the bishop stands in the position of agent, and the religious as passive, as stated above (Question 184, Article 07). Now the agent, even in natural things, the higher it is, is so much the more one, whereas the things that are passive are various. Hence with reason the episcopal state is one, while religious orders are many.
Reply to Objection 4. Confusion is opposed to distinction and order. Accordingly the multitude of religious orders would lead to confusion, if different religious orders were directed to the same end and in the same way, without necessity or utility. Wherefore to prevent this happening it has been wholesomely forbidden to establish a new religious order without the authority of the Sovereign Pontiff.
Source: Summa Theologica (New Advent)