What did the apostles and Old Testament prophets do with the foundation?
The apostles built upon the foundation, which is Jesus Christ. The Old Testament prophets, through what they wrote, added to it. Their contributions fit together for God's eternal purposes, and believers are part and parcel of building on Christ, the hope of salvation.
Les teaches that the foundation is Jesus Christ, and no other foundation can be laid. When that foundation was laid, other apostles built upon it, and the Old Testament prophets and what they wrote added to it. Les explains that everything fits together like a hand in glove for the eternal purposes of God. Paul makes it plain that believers are part and parcel of building on the foundation that is Christ, the hope of salvation.