“Shariputra, why do you think this teaching is called ‘the Sutra of Protection by All Buddhas’? Shariputra, all good men and women who hear this sutra and hold fast to it, and also those who hear the names of those Buddhas [previously mentioned], are protected by all the Buddhas and dwell in the Stage of Non-retrogression for realizing the highest, perfect Enlightenment.”
–The Amitabha Sutra,
(Translation by Hisao Inagaki for the Numata Center for Buddhist Translation and Research)
In addition to the canon of sutras inherited through the earlier Mahasangika school (and another school, the Sarvastivadin), Mahayana Buddhists began composing entirely new sutras in great quantities. In the course of 500 years, the culture had changed in India, moving away from memorizing and verbally reciting teachings to writing them down, and composing new ones.
Compared to the earlier sutras which were supposed to contain the direct teachings of the Buddha, albeit heavily edited for easier memorization and recitation, the newer sutras had a longer, narrative style interspersed with verses. These newer sutras frequently aggregated earlier teachings into a single cohesive narrative with an overarching theme, rather than teachings that were scattered across multiple sutras and collections.
For example, compare this text from a famous early sutra, the Maha-Mangala Sutra (original source):
To loathe more evil and abstain from it, to refrain from intoxicants, and to be steadfast in virtue — this is the greatest blessing….Self-restraint, a holy and chaste life, the perception of the Noble Truths and the realisation of Nibbana — this is the greatest blessing.
With this one from the Sutra on the Buddha of Immeasurable Life, a Mahayana text:
“If in the midst of this, one controls one’s thoughts with single-mindedness, does worthy deeds with proper demeanor, commits no evil, and performs only good, then with the merit and virtue acquired one reaches emancipation and is able to escape from this world, be reborn in heavenly realms, and finally reach Nirvana.
However, the tradition at the time was to attribute teachings to earlier teachers rather than taking credit for oneself, so in the Mahayana Sutras, the teachings are still spoken as if coming from Shakyamuni Buddha, though the historicity is often dubious. In any case, it’s hard to be sure which sutras were composed completely after the time of the Buddha, and which ones were simply expanded from shorter verses into longer narratives. Research is still ongoing.