The scope of Scripture is the message of salvation by Jesus Christ. In that context we speak about the material authority of Scripture. That means: Scripture has authority because of its message. The will of God and the way of salvation is abundantly taught in it. The formal authority of Scripture means that all what the Scripture teaches on whatever topic has divine authority. In article 5 of the Belgic Confession we read: "believing without any doubt all things contained in them."
More than once it has been defended that the Scripture is only infallible in regard to its doctrine of faith and salvation. Men want to hold fast to its material authority but cannot accept its formal authority. Again we must say that it is impossible to separate the saving message of Scripture from all that it contains. The formal and material authority of Scripture presupposes each other and need each other. The formal and material authority of Scripture is two sides of one coin.
The saving message of Scripture is complete reliable because it is revealed to us in the fully inspired, infallible Word of God. We can never separate the content of Scripture form the character of God. What God says and that God speaks to us in Scripture, are completely related to each other. Because of the form given to the Scripture by God (fully and verbally inspired), the Scripture can fulfil the aim for which God has given it to us (to make us wise unto salvation).
The witness of the Holy Spirit
How do we come to the assurance that the Bible is the Word of God? It has been testified in the church during the ages. The Bible itself shows many marks of its Divine origin. Our ultimate assurance that the Bible is the Word of God is not based on the fact that the church receives and approves it as such, but on the inner witness of the Holy Spirit. The Holy Sprits convinces us of our adoption into the family of God. At the same it convinces us of the divine origin, and the divine authority of the message of the Bible.
Paul wrote to the Thessalonians: "For our gospel came not unto you in word only, but also in power, and in the Holy Ghost, and in much assurance; as ye know what manner of men we were among you for your sake (1 Thessalonians 1:5)." In the second chapter of the same epistle we read: "For this cause also thank we God without ceasing, because, when ye received the word of God which ye heard of us, ye received it not as the word of men, but as it is in truth, the word of God, which effectually worketh also in you that believe" (1 Thessalonians 2:13).
A Christian upbringing or Christian witness is as a rule the first way in which we are brought in contact with God’s Word, and is used by God, as the conviction that the Bible is the Word of God. But the witness of other men never can be our ultimate ground of assurance. Our unshakable assurance is based on the witness of the third person of the Holy Trinity in our hearts. Being enlightened by the Holy Spirit we see the majesty of God in his Word. We recognized his voice.
When we try to convince others of the message of the Bible, we can point to the fulfilment of prophecies, the trustworthiness of quite a lot of biblical accounts proved by archaeological excavations, the existence of the Christian church and so on. We can also point to unrest in every human heart and explain it in the light of the biblical message of sin and salvation. We can witness and we are obliged to do it, we only God can really convince and assure people and he does it by his Holy Spirit.