This mystery of Christian solidarity reveals itself also in the spiritual world. We may say of each Catholic what St. Ambrose said of the priesthood: "_Nemo Catholicus sibi_,"--no one is a Catholic for himself alone. By a mysterious law of Divine Providence the conservation and propagation of the faith are, after Divine Grace, largely dependent on the influence of man on man. We are all verily "Our brothers' keepers." We are commissioned by Christ not only to keep the faith but also to hand it down to others, not only to keep its fire burning in our hearts but to spread it, and to fan it into a conflagration. The gift of faith implies the charitable obligation of weaving our belief into our every day life and, through that life and its influence, into the lives of others. The plenitude of some make up for the penury of others. If St. John, to urge the precept of alms-giving, said: "He that hath the substance of this world and shall see his brother in need, and shall shut up his bowels from him: how doth the charity of God abide in him?" (I. Jo. III, 17), with how much more truth cannot the condemnation of the Beloved Apostle be applied to one who, rich in Faith--"that substance of things unseen," makes no effort to help his brother who is deprived of it? Therefore charity, through its spiritual works of mercy, re-establishes the equilibrium in the spiritual realm and stands out as a vital factor in the economy of our religion. To understand rightly this principle and to reduce it to action, is to be a true and ardent apostle. Then, and then only, are we able to say in truth, with the martyr, St. Pacien, "Christian is my name, but Catholic is my surname." How pressing is this obligation to be an apostle, to be truly Catholic, among our non-Catholic brethren? Why should we particularly turn the energies of our zeal to the conversion of non-Catholics? What special claim have they to our prayers? The supernatural element of Faith, often the fruit of a valid baptism, which still lingers in the souls of many non-Catholics; the fact that numbers of them, because they are in good faith, belong thereby to the "Soul of the Church;" the rising tide of indifference and unbelief which is now burying under its water the last remnants of Christianity to be found among the conflicting creeds: these are the predominant motives which, according to the principles of St. Thomas Aquinas, should attract the preference of our zeal. For the order of the charity, says the Holy Doctor,[1] depends on the _relations_ of those we love, to God and to ourselves, and on the _urgency_ of their spiritual needs. By this doctrine, among those outside of the Church, those professing Christianity have the first claim to our apostleship. Therefore missions to non-Catholics, _caeteris paribus_, take precedence over foreign missions.