The Christian must not only accept suffering: he must make it holy. Nothing so easily becomes unholy as suffering.
Merely accepted, suffering does nothing for our souls except, perhaps, to harden them. Endurance alone is no consecration. True asceticism is not a mere cult of fortitude. We can deny ourselves rigorously for the wrong reason and end up pleasing ourselves mightily with our self-denial.
Suffering is consecrated to God by faith--not by faith in suffering, but by faith in God. To accept suffering stoically, to receive the burden of fatal, unavoidable, and incomprehensible necessity and to bear it strongly, is no consecration.
Some men believe in the power and the value of suffering. But their belief is an illusion. Suffering has no power and no value of its own.
It is valuable only as a test of faith. What if our faith fails in the test? Is it good to suffer, then? What if we enter into suffering with a strong faith in suffering, and then discover that suffering destroys us?
To believe in suffering is pride: but to suffer, believing in God, is humility. For pride may tell us that we are strong enough to suffer, that suffering is good for us because we are good. Humility tells us that suffering is an evil which we must always expect to find in our lives because of the evil that is in ourselves. But faith also knows that the mercy of God is given to those who seek Him in suffering, and that by His grace we can overcome evil with good. Suffering, then becomes good by accident, by the good that it enables us to receive more abundantly from the mercy of God. It does not make us good by itself, but it enables us to make ourselves better than we are. Thus, what we consecrate to God in suffering is not out suffering but our selves
No Man Is an Island, Chapter Five,"The Word of the Cross", Thomas Merton 1955
This is a great summary of what makes suffering redemptively of great value in our spiritual transformation: what is evil can contribute to our good if we seek God in our suffering and, thus, receive His mercy all the more. We suffer because of our sarx, because of the evil that is within us and around us. Yet
To the suffering brother or sister Christ discloses and gradually reveals the horizons of the Kingdom of God: the horizons of a world converted to the Creator, of a world free from sin, a world being built on the saving power of love. And slowly but effectively, Christ leads into this world, into this Kingdom of the Father, suffering man, in a certain sense through the very heart of his suffering. For suffering cannot be transformed and changed by a grace from outside, but from within. And Christ through his own salvific suffering is very much present in every human suffering, and can act from within that suffering by the powers of his Spirit of truth, his consoling Spirit.
Apostolic Letter Salvifici Doloris, Section 26, Pope John Paul II 1984
Through our joining our suffering with the suffering of Christ in His Passion and Death, we are led into the Kingdom of God, into the royal nature (basileia) of God, because God is present in our human suffering. To suffer redemptively is to seek God in the midst of our suffering and "to receive more abundantly from the mercy of God" within ourselves, within our hearts. This is not easy.