Objection 1. It would seem that tithes should not be paid to the clergy. Tithes were paid to the Levites in the Old Testament, because they had no portion in the people's possessions, according to Numbers 18:23-24. But in the New Testament the clergy have possessions not only ecclesiastical, but sometimes also patrimonial: moreover they receive first-fruits, and oblations for the living and the dead. Therefore it is unnecessary to pay tithes to them.
Objection 2. Further, it sometimes happens that a man dwells in one parish, and farms in another; or a shepherd may take his flock within the bounds of one parish during one part of the year, and within the bounds of one parish during one part of the year, and within the bounds of another parish during the other part of the year; or he may have his sheepfold in one parish, and graze the sheep in another. Now in all these and similar cases it seems impossible to decide to which clergy the tithes ought to be paid. Therefore it would seem that no fixed tithe ought to be paid to the clergy.
Objection 3. Further, it is the general custom in certain countries for the soldiers to hold the tithes from the Church in fee; and certain religious receive tithes. Therefore seemingly tithes are not due only to those of the clergy who have care of souls.
On the contrary, It is written (Numbers 18:21): "I have given to the sons of Levi all the tithes of Israel for a possession, for the ministry wherewith they serve Me in the Tabernacle." Now the clergy are the successors of the sons of Levi in the New Testament. Therefore tithes are due to the clergy alone.
I answer that, Two things have to be considered with regard to tithes: namely, the right to receive tithes, and the things given in the name of tithes. The right to receive tithes is a spiritual thing, for it arises from the debt in virtue of which the ministers of the altar have a right to the expenses of their ministry, and temporal things are due to those who sow spiritual things. This debt concerns none but the clergy who have care of souls, and so they alone are competent to have this right.
On the other hand the things given in the name of tithes are material, wherefore they may come to be used by anyone, and thus it is that they fall into the hands of the laity.
Reply to Objection 1. In the Old Law, as stated above (1, ad 4), special tithes were earmarked for the assistance of the poor. But in the New Law the tithes are given to the clergy, not only for their own support, but also that the clergy may use them in assisting the poor. Hence they are not unnecessary; indeed Church property, oblations and first-fruits as well as tithes are all necessary for this same purpose.
Reply to Objection 2. Personal tithes are due to the church in whose parish a man dwells, while predial tithes seem more reasonably to belong to the church within whose bounds the land is situated. The law, however, prescribes that in this matter a custom that has obtained for a long time must be observed [Cap. Cum sint, and Cap. Ad apostolicae, de Decimis, etc.]. The shepherd who grazes his flock at different seasons in two parishes, should pay tithe proportionately to both churches. And since the fruit of the flock is derived from the pasture, the tithe of the flock is due to the church in whose lands the flock grazes, rather than to the church on whose land the fold is situated.
Reply to Objection 3. Just as the Church can hand over to a layman the things she receives under the title of tithe, so too can she allow him to receive tithes that are yet to be paid, the right of receiving being reserved to the ministers of the Church. The motive may be either the need of the Church, as when tithes are due to certain soldiers through being granted to them in fee by the Church, or it may be the succoring of the poor; thus certain tithes have been granted by way of alms to certain lay religious, or to those that have no care of souls. Some religious, however, are competent to receive tithes, because they have care of souls.
Source: Summa Theologica (New Advent)