For one thing, one cannot help but take special notice that this was the seventh covenant between God and man, since the number seven has always had a special significance, the number of perfection. The seventh day was the day God rested and commanded to be kept holy, there were the seven eyes of God, the seven feast days of God, seven blessings, seven dispensations, in the Christian world there are seven sacraments, seven cardinal virtues and seven deadly sins, the seventh angel announcing the apocalypse and being the last to punish the world which ended with seven plagues. The Bible is literally riddled with the number seven and it seems only natural that the seventh covenant would stand out even if simply for being the seventh. However, it must also be said that, this was according to the list I learned in school. Some number the covenants differently, so you may take or leave that as you please. What is also interesting and unique about the seventh covenant was that it was the only covenant with man that God made unconditionally. Every other covenant or agreement between God and man followed the pattern of doing something in return for something else. So, God would tell man, if you do this and refrain from doing that, I will do this for you. The seventh covenant was different. There were no conditions, simply a promise from God to King David.
In time, early Christians adopted the position that it was the Church that was the “people of God” and that the Davidic covenant was fulfilled in a spiritual sense by Jesus Christ. He was of the Davidic bloodline and was the eternal King of all and everyone. So, by this reasoning, the covenant remains fulfilled with Christ the King reigning in Heaven, represented on earth by His Church. However, in time, religious thinkers and royal princes alike would start to try to pull the Divine Right of Kings down from the Heavens and apply it to life on earth again. It is understandable that there was some confusion. Christianity was building on the earlier Jewish traditions but clearly most were not Jews and never had been. Even in the New Testament there was already some argument over what Jewish traditions were to be kept and which were no longer relevant. This also came about during the reign of the Roman Empire under monarchs who had nothing to do with Judaism or Christianity. Nor was the imperial monarchy solidly based on bloodline (though it certainly mattered). It was a hereditary succession more often than some think and that was almost always the preferred route but the emperors never technically reigned only because of their bloodline. An emperor did not rule in the name of God (they were not Christians until the latter stages of the empire) but in the name of the Senate and the People of Rome (SPQR). However, the Roman Empire eventually fell, in the west anyway, and new kingdoms rose up to take its place. As these kingdoms rose in power so to did an interest in the Divine Right of Kings.
That being said, there were Catholics who believed in the Divine Right of Kings and Catholic bishops who defended and upheld it. These, however, tended to be outside of the Holy Roman Empire where, it would make less sense anyway as the imperial throne was electoral and not bound to a single family though it eventually became effectively so. Rather, the monarchy most associated with the Divine Right of Kings in the Catholic world was the Kingdom of France, most especially during the zenith of its power and prestige under King Louis XIV, often cited as the quintessential absolute monarch. It also came to be expounded in Great Britain under the Stuart monarchs, starting with King James I who was quite a champion of the Divine Right of Kings, at least in theory (he was a practical enough man to know not to carry things too far with Parliament). King James I championed the idea, as did King Charles I or at least many of his most ardent supporters did and so it was not surprising that his son King James II did as well. It is because of King James II and King Louis XIV in France that many Protestants, particularly in the English-speaking world, came to portray the Divine Right of Kings and even just plain and simple royal absolutism as being a Catholic thing when, of course, the matter was not as simple as that. The Pope, as most know, opposed the King of France and was so adamant about it that he sent extensive financial support to the Protestant Prince of Orange who, though fighting against France, also invaded Britain and overthrew the last Catholic British monarch.
However, the Divine Right of Kings began to fall out of favor because of opposition from both sides of the religious divide. In Catholic countries there was always tension between the monarchs and the Pope. Monarchs resisted Papal foreign policy, Popes resisted interference in the Church from monarchs and so on. A strict reading of the Divine Right of Kings would not necessarily mean this would have to happen. If Popes and Kings both kept to their own sphere it might not have been a problem but neither seemed able to. Bourbon France and later Bourbon Spain were the last bastions of divine right monarchy in many ways and in those countries, Princes of the Blood had the highest status and being “of the blood” really meant something. However, they eventually were lost to the French Revolution in France and even in Spain the absolute monarchy eventually came to an end. Some legitimists in France still carry on some of the basic principles of the Divine Right of Kings but even then it is usually not outright as it would put them in an uncomfortable position with the Church and most are staunch Catholics. In Spain, the Carlists maintained it longer than most would have thought possible but, eventually, they fragmented and those claiming the title today have basically said bloodline no longer matters at all, only ideological adherence.
That highlights one of the main misunderstandings about the Divine Right of Kings which is that it is simply another word for royal absolutism. This is not true as it was more about royal legitimacy than royal power. It meant that there was a sacred bloodline which God had chosen to rule and no one could usurp that rule. It also meant that the monarch answered to God alone and thus was absolute but, as the famous Bishop Jacques Bossuet (usually cited as a proponent of the Divine Right of Kings) wrote, absolute power is not the same as arbitrary power which is another mistake most people make. It is also true that, whether a good thing or not, the Divine Right of Kings never held sway over the whole of Christendom nor was the concept widely supported for very long. It was really never an issue at all in Eastern Europe and in the West it had intermittent support before finally being rejected by Catholics and Protestants alike. Still, it lingered for quite a long time and even in days when no one would have seriously upheld it, it is interesting to note that most royal houses liked to have a genealogical record on hand, showing their lineage back to King David. Even if their position and authority did not depend on it, this was reassuring to have. Unfortunately, the divine right also caused monarchists to become their own worst enemies in some cases as arguments over legitimate bloodlines were pursued so fiercely that some people began to believe, like the French, that a republic would be less divisive -a terrible perception all monarchists should strive to overcome.