What unknown laws were mentioned in earlier Coronation Oaths of England?

I hope to avoid legal discussions (please see final paragraph if you're particularly interested in English legal history), and focus more on the history of the Coronation Oath Act 1688.

Your question: what are the "ancient Laws and Constitutions" (in the Preamble)?

Let's start with, a Preamble to any Act of Parliament is not law (i.e. merely serves as introduction and description to the new legislation). Therefore, the proper way to read a preamble is to use it as an interpretive aid. To do so, however, you have to read the entire Preamble (in toto).

As for the elusive ancient laws and Constitutions stated here, the Preamble of Coronation Oath Act 1688 is not a reference to specific laws or Constitutions per se. If it were in reference to any laws in particular, it would be stated explicitly. In fact, the English common law is a system of laws and conventions that are based on decisions of judges, courts, tribunals and so forth. Hence, this Preamble is referring to the practices of previous Parliaments (including the oaths of previous monarchs, etc).

Two additional points in context of the Glorious Revolution

1. Setting a precedent: The way to understand the reference to "ancient laws" is to see that this particular Act (and the Preamble) is Parliament's demand that all future Kings take the oath, i.e. the final part: "may be in all Times to come taken by the Kings and Queens of this Realme and to Them respectively Adminstred ..." In other words, Parliament is asking the new joint-monarchs (William and Mary) to ignore the previous customs and start anew. The last paragraph in sempaiscuba's answer is the same point here. The details are explained here: Parliamentary history to 1690.

2. The Anglican schism (also known as nonjuring schism): This Act has to be read jointly with the Bill of Right 1688 because the Bill of Rights 1688 required the monarch to make a solemn public declaration of non-belief in the Roman Catholic faith to be made by a new King. And the form of administration of this oath required the King/Queen to adhere to the Protestant faith (source: Institute of Historical Research, University of London, Section 3) - emphasis mine:

Will You to the utmost of Your power Maintaine the Laws of God the true Profession of the Gospell and the Protestant Reformed Religion Established by Law? And will You Preserve unto the Bishops and Clergy of this Realme and to the Churches committed to their Charge all such Rights and Priviledges as by Law doe or shall appertaine unto them or any of them.

By the way, Westminster Parliament has issued a research brief on The Coronation Oath (pdf).

Anticipating comments because of an interest in English legal history, in particular the matter of UK Constitution and Magna Carta, I recommend a short paper by Parliament, but please beware (caveat emptor), it is not a straight-forward topic.

I remember being taught about this when I studied the Stuarts at college. As I recall, the phrase:

" ... ancient Laws and Constitutions at this time unknowne."

meant laws that were no longer recognised in English Law. Now, given the state of many of the public records at that time (many of which had not been stored in ideal conditions!), it is probably also true to say that many of the ancient laws were, indeed, unknown. However, "Unknowne" in this context is simply a synonym for "not recognised".

[The National Archives have produced a podcast that describes the history of the Public Records Office, which includes some information about how and where the records were stored in the past. I'm sometimes surprised that anything has survived at all!]

I do know that some of the earlier oaths survive. The earliest is the oath composed by Archbishop Dunstan for the coronation of Edgar 973 AD:

Three things I promise in Christ's name to the Christian people subject to me:

First, that the church of God and the whole Christian people shall have true peace at all time by our judgement;

Second, that I will forbid extortion and all kinds of wrong-doing to all orders of men;

Third, that I will enjoin equity and mercy in all judgements, so that God, who is kind and merciful, may vouchsafe his mercy to me and to you.

The oath sworn by Edward II in 1307 included the catechism from the Archbishop:

Sire, will you grant an keep and by your oath confirm to the people of England the Laws and customs granted to them by the ancient kings of England, your righteous and godly predecessors, and especially the laws, customs and privileges granted to the clergy and people by the glorious King Edward, your predecessor?

to which the King replied:

I grant and promise.

• [Maitland, 2001, p99]

This is the earliest reference that we know of to "Laws and customs granted to them by the ancient kings of England". A similar reference appears in the text of the coronation oath of Edward IV from 1461, which has also survived.

The Coronation Oath sworn by Henry VIII still survives in the collection of the British Library. It still bears the annotations of the king himself. A digital copy can be viewed online. However, we cannot say for certain whether Henry actually swore the original version of the oath, or the amended version at his coronation in 1509.

A translation and transcription of Henry VIII's Coronation Oath is also available online (although this does rather take away some of the fun of deciphering his handwriting in the original).

As you can see, this again retains the reference to

"... the laws and customs given to them by the previous just and god-fearing kings."

This was used as the basis for the coronation oaths of the subsequent Tudor monarchs, with amendments to the parts of the oath that referred to the Church, according to the religious preferences of the monarch. For example, Thomas Cranmer amended the oath taken by King Edward VI in 1547, so that:

“reformation of the Church could now be enabled by royal prerogative, the king as lawmaker”

The Stuart kings James I and Charles I both kept the form of the coronation oath they had inherited from the Tudors, and after the Restoration, Charles II and James II both took the same oath as Charles I.

Following the Glorious Revolution, however, Parliament felt able to flex its muscles. This led to a series of acts that redefined the English monarchy, beginning with re-writing the coronation oath, set out in the Coronation Oath Act of 1688, and ending with the Bill of Rights, agreed by William and Mary in 1689.

Sources

• Maitland, Frederic William: The Constitutional History of England: A Course of Lectures Delivered, The Lawbook Exchange, 2001