The Scriptures are (i.e. the Bible is) the written testimony of ancient Israel through its prophets (Old Testament) and apostles (New Testament).
These ancient people claimed to have been commissioned by God – the God who had created the universe – to testify to His truth and His ways. You can accept or reject this claim, but there is no reasonable way to read their Scriptures and deny that they make this claim.
Luke 24:25 – Jesus is struck by the disciples reluctance to take seriously all that Israel’s prophets had prophesied. See Acts 13:23 below.
John 4:22 – “salvation is from the Jews” – See also Romans 3:1, 2 below.
Acts 13:23 – “according to promise” – The gospel of Jesus Christ includes not just the testimony of the apostles about the life, death, and resurrection of Jesus but also that all of it was promised by God generations in advance. See also Romans 1:1-3; 1 Corinthians 15:3-4; Titus 1:2; all below.
Romans 1, 1, 2, 3 – “which He promised beforehand through His prophets in the holy Scriptures” – See note on Acts 13:23 above.
Romans 3:1, 2 – “[The Jews] were entrusted with the oracles of God” – This is why Jesus said what He said in John 4:22 above.
1 Corinthians 15:3 – “according to the Scriptures” – See note on Acts 13:23 above.
1 Corinthians 15:4 – “according to the Scriptures” – See note on Acts 13:23 above.
1 Thessalonians 2:13 – “when you received the word of God which you heard from us, you accepted it not as the word of men, but for what it really is, the word of God”
2 Timothy 3:14-17
Titus 1:2 – “which God, who cannot lie, promised long ages ago” – See note on Acts 13:23 above.
James 5:10 – “the prophets who spoke in the name of the Lord” – The prophets did not speak in their own names; rather, they spoke in the name of the Lord who sent them.
2 Peter 1:20, 21 – “men moved by the Holy Spirit spoke from God”
2 Chronicles 20:20 – “put your trust in the LORD your God and you will be established. Put your trust in His prophets and succeed.”
Psalm 118:26 – “Blessed is the one who comes in the name of the LORD” – The ancient Jews came in the name of the Lord of heaven and earth.
Jeremiah 15:16 – “Your words were found” – The words about which Jeremiah is speaking are the words of the Lord. Thus Jeremiah is not claiming to speak in his own name, but rather in the name of the God whose words he had found.