Chi is a concept usually,associated with Taoism and tai chi, not Christianity or Buddhism (except for tantra). However, I have done tai chi for a long time so the concept of chi is relevant for me. I think that chi is a name for something that exists but is not named in Christianity and mainstream Buddhism. First, chi is relational. It exists in the flow between different things, between different parts of your body, between you and another person, and between us and the natural world. Manipulating chi can produce real, measurable, physical effects. This is well studied and I have seen many compelling demonstrations, but nobody has ever been able to physically detect chi. This is because it isn't a thing, it is an emergent effect arising from the dynamic interaction between systems (in my opinion). Chi is the name for this. If chi is manipulated correctly you get interesting effects. However, the only way to do this is to do the right thing at the right time. So chi cannot be summoned or controlled by an act of will, instead you must act in accord with the situation. Therefore, the concept of chi is related to dependent co-arising and skillful means in Buddhism, as well as the ability to act with grace in Christianity. Acting through chi is embodied wisdom that flows along with the moment. Related to this, an important thing about acting through chi is that it cannot be planned, the correct timing must come out of the moment and in order to do this you must be silent internally.
Different religions have different flavors because they are built on the insights of different individuals living in different cultures. Taoism is is particularly good at describing the human experience with flow, being in the moment, and the dynamic relations between all things. Taking the best from each religion (as well as incorporating science) is the key to a sophisticated spiritual life (in my opinion).