The strongest arguments rely on a multiplicity of shared authorities. St. Thomas Aquinas was the master of this. In his early work, Summa Contra Gentiles, he outlined every argument for Christian faith and doctrine using only those authorities which would be shared by a non-Christian audience -- such as natural science and the Greek philosophers. In his later and more famous Summa Theologica, he expanded his earlier writing and developed a fully systematic theology based on authorities common to a Christian audience -- including Scripture and church tradition.
Authorities allow us to make and judge arguments. However, it should be noted that in order to accept an authority, we must evaluate it first, to determine whether it is reliable, or how to rank the value and reliability of multiple authorities. In other words, the process of accepting an authority is itself a judgment, which must be reached on the conclusion of a separate argumentative process, and must therefore rely on authorities itself.
This illustrates one of the more difficult topics in epistemology: the regressive nature of knowledge. Scientists often like to speak of the progressivity of knowledge: one discovery builds on the foundation of another. Yet even in that we see its corollary: if ideas build on each other, then an idea must have come before it. The reasons we give for one argument, rely on the outcome of another previous argument, and so on.
Epistemology is not self-sufficient. In order to conclude anything, we must first agree on something.
If our common foundation were an authority (an outside source of knowledge), then we would fall into a flat contradiction, for our acceptance of that authority would rely on a prior common understanding that such an authority can be trusted. Only personal experience qualifies as evidence not dependent on others, but not even that will suffice: for that must rely implicitly on the reliability of our own experience. Moreover, personal experiences cannot be held in common, so the point is moot from the start.
The very nature of authority points us to an authority outside itself.