In my experience, few things solidify a Catholic against the creeping errors of the modern age better than a solid grounding in the Old Testament. Even in the post-conciliar age, when the importance of biblical study is being emphasized by the Magisterium and at the episcopal level, there are few Catholics who feel comfortable in books such as Leviticus or Deuteronomy, pillars of the Old Testament.
This is understandable; the Beatitudes are much easier reading than the list of the sacrifices in Leviticus 1-7 with their gruesome instructions for removing livers, burning fat, etc. One tends to get more immediate spiritual edification from the Psalter than from the stories of the genocidal warfare of Joshua or the constant plagues visited upon the Israelites as part of God's judgment, as recounted in Exodus and Numbers. I think these "dark" passages of the Old Testament (as the pope called them in his new Post Synodal Exhortation, Verbum Domini) are what the 2008 Synod on the Word of God was getting at when it repeatedly mentioned the "difficulties" of the Old Testament in its Instrumentum Laboris (see here).
Yet, though the Old Testament can be a little tricky to navigate through, this should not put us off, as it was the nourishment of the Apostles and Fathers and beloved by the Saints, who found in its inspired pages an abundance of practical and theological insight. No serious Catholic doubts the general value of the Old Testament, of course, but I want to dwell for a moment on its specific value as an antidote against some of the most pernicious errors of the modern day. We need to dig into the Old Testament and get comfortable in it.
An amazing example is Korah's rebellion in Numbers 16, in which Korah, a Levite, leads a revolt of 250 men against the authority of Moses and Aaron, claiming in astonishingly modern parlance: "Enough from you! The whole community, all of them, are holy; the LORD is in their midst. Why then should you set yourselves over the LORD'S congregation?" (Num. 16:3). They proceed to charge Moses and Aaron with clericalism in reserving the priesthood to themselves and demand that Moses adopt a more community-centered view of the priesthood. Sound familiar? Well, we know how it ends: fire comes forth from the tabernacle and consumes Korah and his band; another group of rebels "went down alive into Sheol" (v.33) when the ground opened up beneath them and swallowed them whole.
How can anyone who is steeped in the Old Testament and is familiar with this story fail to see the very relevant modern principle here? There is a divinely appointed sacerdotal authority (Moses and the sons of Aaron) that is restricted to a certain group. Up comes the rebels, insisting that "the whole community, all of them, are holy; the LORD is in their midst" and demands a democratizing of the hierarchy! God responds by slaying them all. Anybody well-versed in this story now should be forever insulated against any such arguments put forward by modern democratizers of Church authority, since we know authoritatively from the story that God really looks down upon usurpation of sacerdotal authority by outsiders (to out it mildly).
Another example - can anyone who has read the very precise directions for the Temple worship have any doubt that God cares about liturgical details? Reading over the astonishing amount of detail that God commanded for His worship in the Old Testament should inoculate us against the modern idea that how the liturgy is carried out doesn't matter as long as we have a valid Eucharist. If you went into the Old Testament tabernacle and tried that approach, you'd likely have to be carried out!
Can anyone who knows the great care with which the Israelites had to approach the Ark of the Covenant, which was only made of perishable items, should therefor tremble in holy fear at the Eucharist and would probably be disposed about taking their Creator into their hand; after all. Uzzah was struck dead just for touching the box that held the presence of God.
Can anyone who has read of God's scathing warnings against idolatry in the last chapters of Deuteronomy or seen the way in which He punishes this sin (the slaying of the Israelites who worshipped Baal of Peor, for example - Num. 25) ever think for a moment that pagan religion was commendable, or that all religions are bascially good and praiseworthy? Indeed, it is these passages of the Old Testament dealing with idol worship and God's absolute condemnation of it in the most harsh, even violent, terms that come to my mind immediately when I hear stories about nuns in India frequenting Hindu temples, Hindu shrines being set up at Fatima, pagan practices accepted into the Mass in various places due to "inculturation," etc.
Of course, you don't need to be an Old Testament scholar to be opposed to the things I have mentioned above. But the point is that a healthy foundation in the Old Testament will inculcate in one an inherent skepticism towards those modern innovations, since they are so roundly condemned in the writings of the Law and the prophets. The Old Testament gives one a fundamental disposition against these things, such that you cannot even imagine arguing about them because they seem so clearly and unambiguously reprobated in the Old Testament. One cannot thoroughly love the Old Testament on its own terms and be a modernist.
I know that the New Law has superceded the ritual of the Old. But I don't think this is an issue, because I am not suggesting that we treat idolaters the same way they were treated in the Old Law. The command to slay idolaters among the people might not still be valid, but it does give us an insight into how God looks at idolatry, and this insight is valid and timeless. For this reason, every serious Catholic should have a firm grounding in the Old Testament, for it is a most effective remedy against modernist error of every sort.