The first reading is shocking, especially for parents. Who in their right mind would worship a God who demanded the blood sacrifice of the longed-for and promised child? That was a demand that the pagan gods of surrounding countries would make. There is also the fact that the child’s mother, Sarah, who had born Isaac in her old age, was not consulted. It is best perhaps to see this as a parable of the faith of Abraham, who put his complete trust in the Lord. He is, after all, called “our father in faith”. He also foreshadowed the God who gave his only Son (John 3:16). Again, the wording can be misleading; a father giving his son in sacrifice seems harsh. Perhaps we might consider St Ignatius’ image of the the Three Divine Persons looking down on mankind in all its misery and deciding that the Second Person would become human to save humanity (Spiritual Exercises of St Ignatius Loyola: Contemplation on the Incarnation).
Jesus knew very well that his disciples were expecting a triumphant, rather than a suffering, Messiah. It was therefore important for him to give them a vision that would sustain them in the dark days of his crucifixion. Perhaps Peter, James and John were selected because they were seen as leaders in the group of disciples. What they witnessed on the mountain was the history of salvation encapsulated in the figures of the law-giver Moses and the great prophet Elijah, with Jesus as the fulfilment of the promise.