This article sheds light on the unity and continuity between the Christian mystery and the Catholic Social Teaching (CST) and, in a broader scope, between dogmatic and moral theology. It presents, in a synthetic manner, a theological foundation of the CST's principle of solidarity. This study highlights the Trinitarian roots of the principle of solidarity and connects it with creation, redemption, the Eucharist, the communion of saints and eternal life. It concludes that Christian identity inevitably requires a principle like solidarity for the sake of social order. Finally, the article emphasizes the unity between the CST's three main principles (human dignity, solidarity and subsidiarity) to help discern whether their implementation is compatible with Christian identity or not. This article is useful for scholars interested in the Christian foundations of solidarity.