Because they do not understand its profundity,
The Vaibhashika schools deny the Mahayana;
And because they do not believe in nirvana,
Some non-Buddhist schools deny the Hinayana.

Here Shantideva makes two general points. The first is our not understanding something is not a valid basis for saying that it is not a valid teaching. We probably do not hold onto the Hinayana objections to the Mahayana scriptures, but we do quite often reject instructions we do not understand. In fact we do this all the time, usually because we misunderstand what the instruction is saying but assume that the misunderstanding is what is being taught and therefore we reject the instruction based upon a misunderstanding. Or sometimes we give in to the laziness of discouragement where things are difficult and so we give up bothering to try, and in an effort to rationalize our giving up we say that we don’t need this anyways. Middle school students do this all the time with respect to pretty much every subject. They say I will never need this, therefore they don’t need to learn this. The truth is because the subject is hard and they don’t want to do the work, they come up with a rationalization as to why they shouldn’t bother doing so.

The second general point Shantideva makes is just because someone else disagrees with an instruction within our tradition does not mean we ourselves should doubt that instruction. Very often when we hear others disagreeing with parts of our tradition we then generate doubts about those specific instructions, but when other traditions seem to confirm or teach something similar to what is taught in our tradition we take greater confidence in it and think that therefore it is valid and reliable. This is quite common. But Shantideva is pointing out that that is not a valid basis for rejecting a Dharma instruction. If we only believe the instructions that everybody agreed with then those instructions probably would not be very helpful anyways because they would be reflective of deluded ways of thinking that dominate our modern society.

Buddha’s purpose in teaching both the Mahayana and the Hinayana
Was to lead living beings to permanent liberation from the cycle of suffering.
Focusing on this ultimate aim, practitioners of both the Mahayana and the Hinayana
Emphasize the three higher trainings of moral discipline, concentration, and wisdom.

(9.45) Buddha gave his teachings as medicine to cure the disease of the delusions, the cause of all suffering.
Some of his teachings are simple and others are very profound.
If you do not understand his higher, more profound teachings,
You should not simply conclude that they were not taught by Buddha.

(9.46) The great Master Kashyapa gathered many of Buddha’s teachings,
Principally the Perfection of Wisdom Sutras,
Buddha’s Mahayana teachings.
However, the Vaibhashika schools do not understand the profound meaning of the Perfection of Wisdom Sutras;
Thus, they conclude that these Sutras are not Buddha’s teachings.

Here, Shantideva makes the point that Buddha gave different instructions according to the different capacities of living beings. As we are reading through the different Dharma books or attending Dharma teachings, we might hear some instructions which seem completely beyond our capacity. What should we do when this happens? Geshe-la explains in How to Transform your Life that there is no fault in setting aside certain instructions for later when they do seem relevant. For example, for the longest time I did not engage in any completion stage meditations. Every time I heard the instructions or read the books, I thought well that’s interesting, but I didn’t actually ever do anything with the instructions. But now that I’ve started engaging in completion stage meditations, I am ble to go back and revisit things that I learned long ago and try learn how to actually put them into practice.

In the same way, when we encounter instructions that seem beyond our present capacity, we should not feel like we need to force ourselves to learn and understand those things or put them into practice right now. We should focus our efforts on those instructions which seem to us to be of the greatest benefit for where we are actually at in our spiritual development.