How is the solidarity of Christians with other people expressed?
Christians are committed to just societal structures. Part of this is universal access to the material, intellectual, and spiritual goods of this world. Christians also make sure that the dignity of human work is respected, which includes a just wage. Handing on the faith is also an act of solidarity with all mankind.
Solidarity is the practical hallmark of a Christian. Practicing solidarity is not just a command of reason. Jesus Christ, our Lord, identified completely with the poor and the lowly (Mt 25:40). To refuse solidarity with them would be to reject Christ.
Is there a natural law that everyone can know?
If people are to do good and avoid evil, certainty about what is good or evil must be inscribed within them. In fact there is such a moral law that is, so to speak, “natural” to men and can be known in principle by every person by reason.
The natural moral law is valid for everyone. It tells men what fundamental rights and duties they have and thus forms the real foundation for life together in the family, in society, and in the State. Because our natural knowledge is often troubled by sin and human weakness, a person needs God’s help and his revelation in order to stay on the right path.