In the second letter that Paul wrote to Timothy he says:
All Scripture is breathed out by God… (2Tim.3:16)
It is obvious to see that this refers to the Old Testament (OT; 2Tim.3:15-16), which was considered to be God’s word before the New Testament (NT) was written, but does it refer to the NT as well?
The statement that ‘all Scripture is breathed out by God’ is general enough to include both the Old and New Testaments: Paul’s words to Timothy in 2Tim.3:15-16 don’t preclude the NT; Paul just acknowledges that Timothy has known the ‘sacred writings’ from his youth and that he should continue to do so, which could — or would — now include the new ‘sacred writings’ of the NT.
The NT certainly includes itself in the Scriptures. The Gospel of Luke, for example, is referred to as Scripture in the first letter that Paul wrote to Timothy (1Tim.5:18); Paul’s writings are called Scripture in 2Peter3:15-16.
As each new part of the NT was written it would have become accepted by the Christian community because it was written by people they knew were authoritative and, additionally, certain of the Christians had the ‘ability to distinguish between spirits’ (1Cor.12:10) to tell what was really from God and what wasn’t (cf. 1Thess.5:20-21; 1John4:1). In this way the canon of NT Scripture was built up quickly as each new section of the NT was written. Because of this we can also be confident that the writings we have in the NT are genuine Scripture, breathed out by God, and that nothing is missing from the canon.