The involvement of Christianity in law-making began with the Emperor Constantine who declared Christianity to be the official religion of the Roman Empire. This led to Christian bishops, in particular, not just acting in their spiritual role but also performing the administrative functions of helping the poor of their parishes, mediating between parishioners and government, and helping to ensure political stability. Although they did not legislate for the Roman Empire, they did act as advisors.
The main types of law created by the church were what is called "canon law", which is a set of rules concerning the internal workings of the church, including, for example, how bishops and cardinals are chosen, the conditions for divorce, causes for excommunication, and qualifications for the priesthood. There are also specific "rules" governing the conduct of men and women in religious orders (nuns and monks), with each order having its own specific rules.
The main areas that religious and secular law interacted historically were baptisms, marriages, and burials. Before the 19th century, many European countries had quite small civil services, and things like secular birth certificates and civil partnerships did not exist. Instead, churches recorded baptisms, marriages, and deaths.