The principle of solidarity, also articulated in terms of "friendship" or "social charity," is a direct demand of human and Christian brotherhood.
An error, "today abundantly widespread, is disregard for the law of human solidarity and charity, dictated and imposed both by our common origin and by the equality in rational nature of all men, whatever nation they belong to. This law is sealed by the sacrifice of redemption offered by Jesus Christ on the altar of the Cross to his heavenly Father, on behalf of sinful humanity."
Source: Catechism of the Catholic Church (Vatican.va)