Modern people cannot understand, or misunderstand, Jesus's call for faith - and indeed misunderstand what it is that we should have faith in.
In the first place, it needs to be clarified that everybody has a metaphysical system in which they believe - in other words, everybody operates on the basis of some understanding of the basis structure of reality. And this metaphysical belief is not a matter of evidence, because in real life all evidence is ambiguous and there is never enough of it.
(And to deny having a metaphysical system, or to doubt all metaphysical systems... these do not evade the necessity of metaphysics as an implicit basis of life; these are simply different and negative metaphysical systems.)
So that is the primary and inevitable act of faith - not of Christian faith, but basic human faith.
But perhaps the main message of the New Testament, and especially the Gospels, and especially John's Gospel (the one written by Jesus's beloved disciple) is that God is Good, and that God loves us as individual persons.
This is the first basis of Christian faith - the Goodness of God.
The second basis of Christian faith is that God loves us, you and me specifically, because we are a Son or Daughter of God - he loves us in the same kind of way, but perfectly and to the greatest extent, that a Good Father loves his children.
This is the second basis of Christian faith - our Relationship to God.
Because we are God's children, there is something of God's goodness in each of us. This is why it is so important, so often emphasized, that we are His children.
So - faith is made possible and real, because something of God is within us; and it is within us because we are a child of God. This piece of God within us (an hereditary piece of 'God's DNA') constitutes our inner guidance system; and means that the guidance system is potentially valid and certain, because of its provenance: it comes-from and is a part-of the Creator.
In striving for faith - this frame tells us what we should be trying to do. It tells us why faith is so important and how faith can be so strong: faith is important because it is our true inner guidance system; it is strong and strengthening because it is divinely capable of truth, and our lives can rest upon it because of its ultimate certainty.
By faith we can understand that by our mortal lives God is trying, in the most complete and thorough and long termist way he can - and harmonized with all other possible goods - to give us, each. personally... the greatest thing he can possibility give us.
In sum, Christian faith enables us to understand God - not only His power as the Creator (other Gods than the Christian are Creator); but also, and most relevantly, to understand that because God is Creator, is Good, and is Our loving Father; therefore we can trust... Life.
That trust in Life, a trust derived from belief in God's power, nature and love - is Christian faith.
That is what is unique about Christianity; and that is the purpose and necessity of Christian faith.