I believe the core purpose of "pure religion" can be summarized in the verse that says, "Men are that they might have joy." "Love one another", "Love thy neighbor as thyself", "When ye are in the service of your fellow beings...", etc. all are ways to encapsulate and spread joy. Not externally stimulated happiness, but internally generated joy. Therefore, on an individual level, the most important quest in life should be the pursuit of joy - which is accomplished through becoming like Christ.
The problem many people face is that they fail to realize that this type of joy has to be generated internally. It can be sparked by an outside event, but it has to flourish despite all of the external forces that tend to extinguish the spark. It has to be something FELT, not IMAGINED. I know my choice of words in that last sentence might be misconstrued and fuel a tangent I do not want, but I choose it nonetheless. Happiness is an image created by the mind; joy is a feeling generated from somewhere else - what we term the spirit or the soul or the heart.
For what it's worth, I have known quite a few people who are not joyful. Many of them have found ways to be content, satisfied, productive, and even happy in their activities, but they don't carry a sense of internal joy that is unaffected by the external forces around them. I have a friend who described this internal joy as "that special glow".
My observation: That glow comes from the discovery and acceptance of a perspective that lights the soul. Mine came from realizing that I truly am a child of God and have the potential to become like Him. I feel this joy regularly - but I also have been (and continue to be) in situations and circumstances that, in and of themselves, are not joyful in the slightest.
As I have said in the past, if someone else has gained a similar joy that sustains them, and if, therefore, they are not open at all to the Gospel that brought my joy to me, then I will never try to force them to intellectualize "their truth vs. my truth" and risk shattering their joy. Perhaps I believe that my beliefs can bring a "fullness of joy", but they will experience it eventually if I am right, so I focus on helping those without a sense of joy to find one - or expanding on existing joy whenever someone is open to that.