The question in the title of this post at first seems to have no significance. Why does it matter what "faith" is?
The question is very important because faith is how we accept Christianity. Interestingly a large number of Christians try to move past faith into a type of knowledge of Christianity.
The move is incorrect for several reasons. First it is doubtful and likely impossible to build a fool proof logical proof to the existence of God.
Second the bible gives the end date of faith and that is upon entrance into heaven. To live by faith is not irrational, but we must live by spiritual sight and not mathematical proof or argumentation.
Interestingly today mathematics, science, and argumentation are seen as having a higher view then spiritual sight. This of course has large scale consequences on humanity.
Mathematics and science are a set of tools and a poor worldview. For example much of the modern morality related to sexuality has come about because of a lack of religion.
There is no mathematical or scientific reasons to support traditional marriage or even the concept of marriage at all.
In the end we find most of the important issues in life cannot be solved by a calculation or measurement or test.
Maybe science could answer lives big question if the world was purely material. The reality is the world has a spiritual component. It cannot fully be tested.
What is wrong with the modern morality related to sexuality has a spiritual basis, not a scientific one. But the ultimate things are spiritual in nature.
It is of course true that we will have a physical body in heaven and Jesus ascended as fully God and fully man into heaven. So we should not degrade the spiritual world.
However the ultimate decision each person faces is spiritual. Will he trust that Jesus is who he says he is and respond in faith.