The Gospels are considered to be both theological and historical, theology begins with the assumption that the divine exists in some form, such as in physical, supernatural, mental, or social realities, and that evidence for and about it may be found via personal spiritual experiences and/or historical records of such experiences as documented by others.
The study of these assumptions is not part of theology proper, but is found in the philosophy of religion, and increasingly through the psychology of religion and neurotheology. Theology then aims to structure and understand these experiences and concepts, and to use them to derive normative prescriptions for how to live our lives. Theologians use various forms of analysis and argument (experiential, philosophical, ethnographic, historical, and others) to help understand, explain, test, critique, defend or promote any of myriad religious topics which the Gospels are one.
As to the question of the Gospels being historical, to prove this scholars turn to external sources, including the testimony of early church leaders, writers outside the church (mainly Jewish and Greco-Roman historians) who would have been more likely to have criticized the early churches, and to archaeological evidence.
When judging the historical reliability of the gospels, scholars ask if the accounts in the gospels are, when judged using normal standards that historians use on other ancient writings, reliable or not. The original gospel works were accurate eyewitness accounts, and whether those original versions have been transmitted accurately through the ages to us. In evaluating the historical reliability of the Gospels, scholars consider a number of factors. These include authorship and date of composition, intention and genre, gospel sources and oral tradition, textual criticism, and historical authenticity of specific sayings and narrative events.
The criterion of dissimilarity argues that if a saying or action is dissimilar to, or contrary to, the views of Judaism in the context of Jesus or the views of the early church, then it can more confidently be regarded as an authentic saying or action of Jesus.
The criterion of embarrassment holds that the authors of the gospels had no reason to invent embarrassing incidents such as the denial of Jesus by Peter, or the fleeing of Jesus' followers after his arrest, and therefore such details would likely not have been included unless they were true.
The criterion of multiple attestation says that when two or more independent sources present similar or consistent accounts, it is more likely that the accounts are accurate reports of events or that they are reporting a tradition which pre-dates the sources themselves.
15 One witness shall not rise up against a man for any iniquity, or for any sin, in any sin that he sinneth: at the mouth of two witnesses, or at the mouth of three witnesses, shall the matter be established.
16 But if he will not hear thee, then take with thee one or two more, that in the mouth of two or three witnesses every word may be established.
The criterion of cultural and historical congruency says that a source is less credible if the account contradicts known historical facts, or if it conflicts with cultural practices common in the period in question. It is, therefore, more credible if it agrees with those known facts.
Through linguistic criteria a number of conclusions can be drawn. The criterion of "Aramaisms" as it is often referred holds that if a saying of Jesus has Aramaic roots, reflecting Jesus' Palestinian context, the saying is more likely to be authentic. These methods and others have been used in order to determine the accuracy of all biblical text for hundreds and even thousands of years.
The Bible is used today in many occasions to validate historical records through archeological research.