The active life of the Church grows out of the sacraments instituted by Jesus Christ. In the sacraments we experience an outpouring of God’s grace into our lives, they are our life in the Church. 

When Saint Paul wrote his first letter to the Corinthians he said that they should consider him (and the other apostles) “servants of Christ, and stewards of the mysteries of God” (1 Cor 4:1).   When the word mystery was translated from Greek into Latin, it was normally translated as sacramentum; thus the sacraments are the mysteries of Christianity.