well i read the bible and see jesus and the apostles and others talk about the scriptures and at the time they are speaking and writing there are no other scriptures they could be referring to but the law and the prophets. its really basic.
from the beginning of my conversion from judeochristianity to following the lord jesus i could see that the "old testament" was the word of god - the law and the prophets, the torah - and that jesus, the gospels and the epistles were the explanatory notes, the proper interpretation of gods word manifest in word and deed.
thats not just baby's milk, thats the colostrum of the first sucklings. its that foundational.