How does Dante get the point across (in Dante's Inferno) that reason is not enough to help you in your spiritual journey?
The answer to this question depends fully upon a reader's understanding of the text and what message Dante is trying to provide the reader with.
That being said, in Dante's Inferno, Dante is trying to inform readers that reason is not enough to help one on their own personal spiritual journey. In order to illustrate this fact, one must look at the text in an abstract way.
The point about one's spiritual journey not solely relying on reason is an important one. This idea is compacted by the fact that, at some points in life, reason does not help a person at all. Instead, one must come to realize that life is simply one's interpretation of what is happening around them. In order to succeed in a spiritual journey, one must (at times) leave everything else behind and not overanalyze life itself.
At one point, Dante shows pity for those whom have been damned to live all of eternity in hell. If one were to use reason, they would simply justify that the punishment fits the sin and they would not have pity for (using reason) the sinner.
Basically, one cannot live on reason alone. There are times where emotion needs to take over to help one through the difficult phases in life. Therefore, one must have the perfect balance of both reason and emotion in order to succeed in making their spiritual journey.