Are the sacraments necessary? Yes, says St. Thomas. He proposes three reasons for this necessity:
The first is taken from the condition of human nature which is such that it has to be led by things corporeal and sensible to things spiritual and intelligible. Now it belongs to Divine providence to provide for each one according as its condition requires. Divine wisdom, therefore, fittingly provides man with means of salvation, in the shape of corporeal and sensible signs that are called sacraments.
So sacraments are necessary for us because of the kind of beings we are, being corporeal and spiritual.
The second reason is taken from the state of man who in sinning subjected himself by his affections to corporeal things. Now the healing remedy should be given to a man so as to reach the part affected by disease. Consequently it was fitting that God should provide man with a spiritual medicine by means of certain corporeal signs; for if man were offered spiritual things without a veil, his mind being taken up with the material world would be unable to apply itself to them.
Because we are prone to giving the things of this world more attention than we ought, it’s fitting, he says, that by means of visible signs we receive the graces necessary to overturn this earthly fixation.
The third reason is taken from the fact that man is prone to direct his activity chiefly towards material things. Lest, therefore, it should be too hard for man to be drawn away entirely from bodily actions, bodily exercise was offered to him in the sacraments, by which he might be trained to avoid superstitious practices, consisting in the worship of demons, and all manner of harmful action, consisting in sinful deeds.
It follows, therefore, that through the institution of the sacraments man, consistently with his nature, is instructed through sensible things; he is humbled, through confessing that he is subject to corporeal things, seeing that he receives assistance through them: and he is even preserved from bodily hurt, by the healthy exercise of the sacraments. [ST III Q61 A1]
Why did God give us the sacraments? It wasn’t just some arbitrary thing: “You’ll do this because I say so.” Of course that would be sufficient, but we are rational beings and God is rational too. No, there are reasons for the sacraments. God wasn’t acting without good reason when He ordained them for us; it wasn’t mere arbitrariness.
We need the sacraments. We need them to be tangible, because we are corporeal ourselves. And God’s love for us is seen in the fact that He gives us physical signs, and that by means of them He gives us grace.
I don’t really know if Protestants would countenance such an argument as this one (from necessity grounded in human nature), but I don’t see why they wouldn’t. At the very least, though, they ought to concede that the gift of sacraments is something God gives us for a reason. But if they are willing to concede this, then I should think that they should also be willing to grant that the proper form and matter of the sacraments is likewise not merely arbitrary on God’s part, and that consequently these also are not matters of indifference.
And yet Protestants have disagreed about these questions from the very beginning. This was a wedge issue for me, with the ultimate outcome being my reception into the Catholic Church. If the Holy Spirit leads the individual Christian to receive divine truth from Scripture as the Protestant claims, then it is quite frankly impossible that they should disagree amongst themselves about it. But they do disagree about the sacraments. Consequently their claims as to how the Holy Spirit works among them are patently false. Consequently there is no reason, on the Protestant’s own terms, to believe anything that he says about what the Bible teaches.