Fr. Jordan Aumann O.P.:
Offering Oneself as Victim. It would seem that it is impossible to go further in love of the Cross than to prefer sorrow to pleasure. Nevertheless, there is still another more perfect degree in the love of suffering: the act of offering oneself as a victim of expiation for the sins of the world. At the very outset, we must insist that this sublime act is completely above the ordinary way of grace. It would be a terrible presumption for a beginner or an imperfectly purified soul to place itself in this state. "To be called a victim is easy and it pleases self-love, but truly to be a victim demands a purity, a detachment from creatures, and a heroic abandonment to all kinds of suffering, to humiliation, to ineffable obscurity, that I would consider it either foolish or miraculous if one who is at the beginning of the spiritual life should attempt to do that which the divine Master did not do except by degrees.
The theological basis of offering oneself as a victim of expiation for the salvation of souls or for any other supernatural motive such as reparation for the glory of God, liberating the souls in purgatory, attracting the divine mercy to the Church, the priesthood, one's country, or a particular soul, is the supernatural solidarity established by God among the members of the Mystical Body of Christ, whether actual or potential. Presupposing the solidarity in Christ that is common to all Christians, God selects certain holy souls, and particularly those who have offered themselves knowingly for this work, so that by their merits and sacrifices they may contribute to the application of the merits of the redemption by Christ. A typical example of this can be found in St. Catherine of Siena, whose most ardent desire was to give her life for the Church. "The only cause of my death," said the saint, "is my zeal for the Church of God, which devours and consumes me. Accept, O Lord, the sacrifices of my life for the Mystical Body of thy holy Church." She was also a victim soul for particular individuals. Other examples of victim souls are St. Thérèse of Lisieux, St. Gemma Galgani, and Sister Elizabeth of the Trinity.
In practice, the offering of oneself as a victim for souls should never be permitted except to souls of whom the Holy Spirit asks it with a persistent and irresistible motion of grace. It should be noted that, rather than contributing to the sanctification of the individual (although it does add something), this particular act is ordained to the spiritual benefit of others. The soul that would give itself in this way for the salvation of others must itself be intimately united with God and must have traveled a long way toward its own perfection in charity. It must be a soul well schooled in suffering and even have a thirst for suffering. Under these conditions the spiritual director could prudently permit a soul to make this oblation of self as a victim soul. Then, if God accepts the offering, the soul can become a faithful reproduction of the divine Martyr of Calvary. (Source