There are no justified beliefs, i.e., you can never be sure that something is true.
Beliefs can only be justified in relation to other beliefs.

Therefore, all objective knowledge comes about by persuasion that explanations for observations are not wrong. Since no belief can be justified or sure, all objective knowledge is (and should be) open to criticism. Objective knowledge is built by observation, thought, and explanation, and is improved upon by criticism. All knowledge originates from the same source as dreams. 

Conjecture and guesswork are the origin of knowledge but also a source of error. Only by criticizing our guesses and failing to find faults or flaws in them can we provisionally accept them to be true. 

Although all of our perception (subjective experience) comes from our mind, the guesswork in what we perceive is tested against input from our senses–again, our acceptance of external reality is and only ever can be provisional–but from the combination of our conjectures about the external world and the input from our senses we can develop a ‘waking dream’ that corresponds to reality. We gain control of this dream by controlling the corresponding aspects of external reality.

There is an objective world–but our only contact with it is through our perception which can be flawed. [Is it possible for our perception to ever not be flawed? Surely it is not possible for us to be certain that it is so.] The reality that we inhabit is a perpendicular reality of subjective experience–woven together by our minds based on conjectures of what is real. The only point of contact between our subjective world and the objective world is what we experience as the present moment.

Note: all thoughts and arguments here are paraphrased from David Deutsch’s Beginning of Infinity

What does it mean when someone says they are, “spiritual but not religious”? What elements of human experience are they talking about when they use that word? And why is that the situation of so many modern young people, that they eschew the religious institutions of their parents and grandparents, but still desire to keep some essence of ‘spirituality’ in their lives?

Spiritual = Spirit + Ritual

Everything that we understand we come to know through experience. There are two types of experience that can lead to understanding–direct experience, and belief in something that is conveyed to us symbolically.
Direct, pre-linguistic experience allows us to know things in a way that we often find difficult to convey or describe because language is a flawed tool. However, the use of language (defined broadly as any symbolic communication) is the only way in which we can convey the knowledge we have gained through direct experience to others.

Language involves the use of words, which we use to point at terms. Terms are the meaning which language approximates through the use of words. The use of language allows us to come to terms with another person–to understand one another and ensure that we are talking about the same thing when we use the same words.

The problem is that because words are symbols, they are not the thing itself. Our use of language runs ashore as we get deeper into the metaphysical weeds. The more fundamental and abstract the concepts become the more we see language fall short as a modality for both communicating our understanding to others, or coming closer to the truth through the use of language.