How do you know that this person is the person you should marry? How do you know that this college will be a good fit for you? How do you know that this is what you must do with your working career to be fulfilled while making a living? How do you know with certainty about anything?
In these instances and many others in our lives, "we walk by faith and not by sight." We take a "leap of faith" with no guarantees.
As far as faith in our spiritual life, we believe not in a set of doctrines, dogmas, and suppositions, but in a person. It's not so much "I believe in." as "I have faith in you: Jesus Christ and the Father and the Holy Spirit." That's why we have to get to know God rather than things about God, to experience God, to sense God's presence around us.
That is what Jesus means by the Kingdom of God. He wasn't talking about some future event after death but a living experience of the reality of God in our lives right now, to let that awareness grow little by little, almost imperceptive, until we can be at rest for awhile, confident that there is always something more something deeper and happier, eventually leading to the full experience of God: eternal life.
-Fr. Sas, June 14 Homily
The 11th Sunday in Ordinary Time