The questions discussed in both religion and philosophy tend to be very much alike. Both religion and philosophy wrestle with problems like: What is good? What does it mean to live a good life? What is the nature of reality? Why are we here and what should we be doing? How should we treat each other? What is really most important in life?
Clearly, then, there are enough similarities that religions can be philosophical (but need not be) and philosophies can be religious (but again need not be). Does this mean that we simply have two different words for the same fundamental concept? No; there are some real differences between religion and philosophy which warrant considering them to be two different types of systems even though they overlap in places.
To begin with, of the two only religions have rituals. In religions, there are ceremonies for important life events (birth, death, marriage, etc.) and for important times of the year (days commemorating spring, harvest, etc.). Philosophies, however, do not have their adherents engage in ritualistic actions. Students do not have to ritually wash their hands before studying Hegel and professors do not celebrate a “Utilitarian Day” every year.
Another difference is the fact that philosophy tends to emphasize just the use of reason and critical thinking whereas religions may make use of reason, but at the very least they also rely on faith, or even use faith to the exclusion of reason. Granted, there are any number of philosophers who have argued that reason alone cannot discover truth or who have tried to describe the limitations of reason in some manner — but that isn’t the quite the same thing.
You won’t find Hegel, Kant or Russell saying that their philosophies are revelations from a god or that their work should be taken on faith. Instead, they base their philosophies on rational arguments — those arguments may not also prove valid or successful, but it is the effort which differentiates their work from religion. In religion, and even in religious philosophy, reasoned arguments are ultimately traced back to some basic faith in God, gods, or religious principles which have been discovered in some revelation.
A separation between the sacred and the profane is something else lacking in philosophy. Certainly philosophers discuss the phenomena of religious awe, feelings of mystery, and the importance of sacred objects, but that is very different from having feelings of awe and mystery around such objects within philosophy. Many religions teach adherents to revere sacred scriptures, but no one teaches students to revere the collected notes of William James.
Finally, most religions tend to include some sort of belief in what can only be described as the “miraculous” — events which either defy normal explanation or which are, in principal, outside the boundaries of what should occur in our universe. Miracles may not play a very large role in every religion, but they are a common feature which you don’t find in philosophy. Nietzsche wasn’t born of a virgin, no angels appeared to announce the conception of Sartre, and Hume didn’t make the lame walk again.
The fact that religion and philosophy are distinct does not mean that they are entirely separate. Because they both address many of the same issues, it isn’t uncommon for a person to be engaged in both religion and philosophy simultaneously. They may refer to their activity with only one term and their choice of which term to use may reveal quite a lot about their individual perspective on life; nevertheless, it is important to keep their distinctness in mind when considering them.