John Owen on the tithe:

“There is great inquiry usually made on this place, whether tithes
be due by the light of nature, or at least by such a moral-positive
command of God as should be perpetually obligatory unto all worshippers
unto the end of the world. This many contend for, and
the principal reasons which they plead from the Scripture are these:
1. That tithes were paid before the law as well as under the law;
and what was so observed in the worship of God,—namely, that
being in usage before the law, and confirmed by the law,—is originally
of the law of nature, and could have no other fountain. 
2. Our Lord Jesus Christ himself, speaking of tithing mint and cummin,
approveth of it, affirming that those things ought not to be omitted,
though the most inferior instance that could be given of the duty.
3. He seems in like manner to have respect thereunto, when he
commands to “give unto Caesar the things that are Caesar’s, and
unto God the things that are God’s,” which were the tithes; the
law concerning them being thereby confirmed, which proves it not
to be ceremonial. And this some men judge to be a certain argument
of that which is moral and unalterable,—namely, the appointed
usage of i t before the law, under the law, and under the gospel after
the expiration of the law of ceremonies, or ” the law of commandments
contained in ordinances.” And it seems so to be, if there be
the same reason of the law or command in all these seasons; for
otherwise it is not so.”

Reformed Theology at Semper Reformanda