Historically, Abraham is the 'first' man that God, revealing himself, calls to faith: in fact, no one can achieve a firm and consistent faith without an interior 'illumination' of God, a 'word' that God, in many forms and in different situations, sows in the heart of man.
What is 'faith'? it is a 'trust' given to a person. The most immediate image of faith is that of children who trust themselves in the arms of their parents, but our trust as adults is also based on faith, when we rely on the ability of those who carry a plane or drive a bus, or the trust in our doctor or school teacher. We make acts of faith in others every day, because this is the structure of human relationships: no one can live alone. Faith is therefore 'trusting'.
Teresa of the Child Jesus, even amidst pain and terrible nights of interior darkness, on the path of her spiritual childhood, will trustfully entrust herself into the arms of the Father in whom she is certain of finding the response of love. Thus Teresa of Calcutta, thus Charles de Foucauld, thus the great Protestant theologian Bonhoeffer killed by the Nazis after long years of imprisonment, thus the theologian daughter of Abraham and the Church, Edith Stein, also a victim of the holocaust.
Faith is also and above all adherence to the mystery of God, and it is often a struggle with the silence of God. A silence to which God asks for our trusting adherence. Here I should remember, in my long pastoral experience as a parish priest among the people, the intense, innocent, heroic and silent faith of many mothers and fathers: without theology, but true theologians, because they are faithful interpreters of God's plan in their lives and within their suffering and hope.
Without this faith of our people, the Church could not exist; our faith as priests or pastors is often torn by doubts, betrayals, uncertainties: the faith of our people is luminous, silent, substantial, rocky: it is the 'amen' with which the Bible often describes the 'yes ' to God.
Faith in God is our response to his plans; it is our trust in God, because God is faithful, as seen on every page of the Bible. Abraham, to refer again to Saint Paul, is "Father of all those who believe" (Rm 4,11): he truly 'entrusted himself totally' to God, as Scripture reveals by presenting the 'father' and 'founder' of the Jewish people. Of course, even when faith is total, it is never certain and absolute in a fideistic way: it also has bases of rationality: I understand and accept with my intelligence that God exists and can call me, giving meaning to my existence. Faith is an act of understanding a gift or calling that God creates in my life. There is an entire chapter by the author of the letter to the Hebrews which deals at length with how to understand the discussion on faith: it is our 'yes to God', even when this adhesion remains obscure to us.
It is in the 2nd chapter of the Letter to the Hebrews, which everyone can read and where the faith of many characters in the Old Testament is described, where the author gives the definition of 'faith': "the foundation of what is hoped for, and proof of that which cannot be seen" (Heb 11,1:XNUMX). Precisely in this certainty in what 'we hope for', despite 'not seeing it', we understand how faith draws its roots both in our intelligence and in the conscience enlightened by God.
It is a gift that rationalizes the meaning of our existence. Thus it is said of Abraham: "By faith, Abraham, called by God, obeyed by leaving for a place that he was to receive as an inheritance, and left without knowing where he was going" (Heb 2,8). Thus Sarah, Abraham's elderly wife, 'by faith', late in life, became the mother of Isaac (see 2,11).
Faith is therefore born when we experience the 'voice' of God who calls. In faith we entrust our 'yes' to the word of God, and therefore faith is always preceded by the action of what we will call the 'grace' of God. No one can believe on his own if God does not insinuate himself into our spiritual depths. the light and strength of his word.
In this light, it seems to me that there are three decisive moments in the story of Abraham in the face of God who speaks and which the book of Genesis hands down to us: God's order to Abraham to leave his land and the promise of great descendants (Gen 12, 1-4), the divine promises and the alliance (ch. 15), the obedience of Abraham with the sacrifice of Isaac (ch. 22).

1. Abraham is a nomad originally from the city of Haran in Mesopotamia, of pagan origins, willing to undertake a long journey towards an unknown land which is Canaan, following God's invitation to 'leave'.
The Yahwist text of this story is sparse, essential and decisive.   
Gen 12,1:4-XNUMX: “The Lord said to Abram: 'Go from your country, and from your relatives, and from your father's house, to the land that I will show you. I will make you a great nation and I will bless you... and in you all the families of the earth will say they are blessed. And then Abram left, as the Lord had commanded him...".
This is the 'faith' of Abraham which will then make the whole history of Israel speak for itself: I want to underline once again that we can speak of faith, in Abraham of absolute faith, when there is, previously, the 'word of God' which it prevents us, challenges us and pushes us to choose. Biblical faith indicates our 'yes' to God.
I am struck by this decisive verb in Abraham's faith: Abraham 'departed' (v.4).
It doesn't matter 'where', you don't yet know the destination, you don't make any predictions about the journey. 'Leave, go, go out', seem to be the biblical verbs that best describe the history of faith: this will also be the case for Moses in bringing the Jewish people out of Egypt, in the book of Exodus, as will the prophets Isaiah and Jeremiah, taken away from their quiet life to an arduous and difficult mission among the people, this is how many missionaries and witnesses of the Gospel acted in the history of Christianity. The willingness to 'depart', to leave our securities, our affections, our own projects, is often at the root of the mystery with which God acts in our history. Jesus carries out the project of the salvation of the world by allowing himself to be guided at every moment by the will of the Father, as the fourth gospel of John testifies.
Abraham's departure, as an act of absolute faith, however does not eliminate the use of his will and intelligence. And this also happens in our existence of faith: even our choices are often examined not so much by opportunity or simple obedience, but are lived in the depths of our conscience which makes God's proposal appear great and generous, no less fascinating than promises of God to our father Abraham.
In this sense, faith is always an abandonment to the 'word' that God interiorly reveals to us: faith is therefore a response to God who calls. We could still say: faith is the revelation in me of the loving will of God who, in love, plans my life. A project that often goes through suffering and persecution.