Capacity is determined by what is attempting to enter the vessel and the vessel's "space" to hold what is entering. For example, if we have a used jar of peanut butter that has not been scraped clean and washed out, there are remnants of peanut butter in there. For all intents and purposes, we'd say the peanut butter jar was empty - we can easily cast it away as no longer of use to us. No matter how much we attempt to fill that "empty" jar with something else (like jam), that jar would never be totally full of the jam because it still was partially filled with the peanut butter. It may not be much peanut butter, but it still affects the taste of the jam and it takes up space that the jam would like to have enjoyed. Christ is kind of like the jam in our lives - wanting to fill the empty spaces of our lives, but he is competing with the peanut butter - remnants of our old ways of doing things, responding to things, and things we have held onto that we just don't want to part with.
The unspiritual self, just as it is by nature, can't receive the gifts of God's Spirit. There's no capacity for them. They seem like so much silliness. Spirit can be known only by spirit—God's Spirit and our spirits in open communion. Spiritually alive, we have access to everything God's Spirit is doing, and can't be judged by unspiritual critics. Isaiah's question, "Is there anyone around who knows God's Spirit, anyone who knows what he is doing?" has been answered: Christ knows, and we have Christ's Spirit. (I Corinthians 2:14-16)
Capacity is defined as the ability to both receive and contain. The part of our 'self' that might be labeled as un-spiritual cannot receive the gifts of God's Spirit - things like truth, wisdom, and insight into God's ways. Why? To our human nature, these things are silly - they just don't make sense - they are counter-intuitive for us. For example, if you are an engineer and you are speaking with another engineer about how something is constructed (like a bridge), the force it can withstand under pressure, and the specifics of its design, you probably both understand each other. If you are like me, you are thinking, "I just want to cross that bridge - I don't really need to know how it is made in order to do that, do I?" The "details" seem silly to me because they don't "apply" to me - I have no interest in receiving that detail - I want merely to cross that bridge! Many times all we want is to 'cross some bridge' from un-spiritual into spiritual, without much thought as to how much of the un-spiritual is still within us, making it harder for us to embrace the spiritual.
To the one who has not invited the Spirit of God into their lives, the truths shared are nice, but they have no real relevance. They are not received because there is no "use" for them. To the one who has invited the Spirit of God to oversee their lives, giving constant guidance and tutelage, those truths, no matter how small are like that lovely tasting jam - they ooze into every crevasse they have access to and begin to affect that space. The challenge comes in getting all the "peanut butter" out of our "jars"! We cannot contain all God wants us to contain until we make way to have the "vessel" fully cleaned out. We sometimes try to embrace spiritual truths without really allowing the Spirit to deal with the things that need to be removed / cleaned up along the way. Try washing an "empty" jar that has contained peanut butter - that stuff gets into the tiniest nooks and crannies of that jar - making it hard to get all the peanut butter out on our own! You really have to work hard to get it completely clean (unless you have a dishwasher that does the work for you!). That is how it is when we "try" to clean ourselves up after coming to Christ - attempting to deal with the things that entice us to make wrong choices, but they are just hanging on to us like they belong there within us.
The key is to allow the one who has the ability to thoroughly clean the vessel to do that work - just as we'd rely on the dishwasher to remove the peanut butter! If we want capacity for the things of God, we need his Spirit to clean out our vessel of those things that "take up space" without really serving any purpose anymore. I cannot say what those may be for you - but I know that his Spirit will be faithful to point them out and remove them when he is given full access to your life. So, you might want to ask this question: Are you settling for "some" jam with a little of your left-over peanut butter? Or are you desirous of only pure jam in your vessel? If you choose the pure jam, you will need to turn the vessel over to him in order to allow it to be emptied so that your capacity is expanded to receive it fully. God wants full access - not to compete with the "left-overs" of our live's choices and things we have trusted in at some point in the past. Just sayin!