The answer is twofold. We know of God through general or natural revelation and through his verbal revelation which has been recorded in the Bible.
General revelation includes our reason and conscience (“the light of nature”), the creation, and his active work in history governing the world. From these we know that God exists, we know something of his kindness and wrath, and we know something of his moral will. However we need more than this.
We need verbal revelation in order to know of his Salvation through Jesus Christ and how he desires those who would be saved to responds to him in faith, worship, and in holy lives. He has therefore given us verbal revelation that has been recorded in the bible.
This written revelation is not only a need but a gift of our loving Lord. The confession reminds us that through a written revelation the truth is better kept and conveyed and that God’s people are established and comforted with the solid truth of his Word as they fight against their own sin, the world, and the spiritual forces of evil.
Finally the confession reminds us that God has given us all the inspired words that we need. The written revelation we have in necessary AND SUFFICIENT. We need and should not look for new revelations. With the revelation of Christ and the inspired explanations of these things given by the apostles God’s revelation has come to completion. Now his Church is built on the solid rock of his Word.
Ephesians 2:20 tells us that the household of God is, “…built on the foundation of the apostles and prophets, Christ Jesus himself being the cornerstone in whom the whole structure being joined together, grows into a holy temple in the Lord.”