On p. 9 of the Zaccheus Press edition of A Key to the Doctrine of the Eucharist, Abbot Vonier writes:

For the Puritain, faith is not in need of any help or adjuncts. Yet the reason given by Catholic theologians for the presence in the Christian dispensation of these external signs of internal faith [i.e., the sacraments] are chiefly psychological; man's nature being what it is, sacraments are indispensible to a full life of faith.

There are many arguments against the Catholic faith that, in one way or another, boil down to rejecting the claim, "Man's nature being what it is, sacraments are indispensible to a full life of faith."

Abbot Vonier points out one argument, that of the Puritain: "Faith needs no help." That may be true, but it isn't faith that is to be saved, it's man, and man can use all the help he can get.

Another argument pivots on the word "indispensible." God is not bound by the sacraments, so they aren't indispensible in the literal sense that salvation is strictly impossible without them (setting aside the question of how sacramental baptism by desire actually is). But they are indispensible given that man's nature is as it is:

Saint Thomas gives a threefold reason for the institution of the sacraments; but this threefold reason is really one -- man's psychology. However, the three factors are firstly, the condition of man's nature, being a composite of spirit and sense; secondly, man's estate, which is slavedom to material things and only to be remedied by the spiritual power inside the material thing; thirdly, man's activities, so prone to go astray in external interests, finding in the sacraments a true bodily exercise which works out for salvation.

As St. Thomas writes in answer to an objection in the same article Abbot Vonier references:

God's grace is a sufficient cause of man's salvation. But God gives grace to man in a way which is suitable to him. Hence it is that man needs the sacraments that he may obtain grace.

Some would object that the sacraments are suitable to man. Various strains of Manichaeism are still around, even within Christianity, that in effect deny that the spiritual can be expressed or signified by the material. There is something of a passive Cartesian dualism that forgets, if it ever knew, that we really are souls and bodies, not souls temporarily stuck in bodies; have you ever met anyone who thinks we become angels after we die? A healthy faith in the Incarnation and the Resurrection resist this error.

Then there are those who deny there is such a thing as human nature. Their difficulties with the Faith neither start nor finish with the sacraments, and so discussions with them shouldn't either.

It's those Protestants, or even those poorly catechized Catholics, who regard the sacraments as something somehow extra, and so extraneous, whom I would most like to exhort. Jesus tells us He came that we might have life in abundance. To prune back that life to the barest minimum, to keep it at subsistence level lest we be guilty of some sort of excess, is, in a word, un-Biblical.

A full life of faith is what Jesus preached, what the Apostles wrote of. It is a participation, even now, in the life of the Holy Trinity. Half-measures and bare subsistence are not God's ways, nor should they be ours. And because they are available in the Christian dispensation, a full life of faith requires, demands, yearns for, and embraces the sacraments.