Behold how Christ is the foundation of the church and the apostles are the foundations! Christ is by a figure of speech - antonomastice - the foundation because the edifice of the church begins from him and is finished in him and through him. But the prophets and apostles are the foundations because their authority bears up our weakness.
A precept or command is a general teaching of God, obligating every man under pain of mortal sin - namely, in cases in which he has fallen away from the command. Hence, the saints who for a period of their life lived hypocritically sinned mortally for that period. So also the damned, by persistent false living, sin persistently in Hell.