But I know that those who trust only in the teaching of Hinayana will not be agree with that. They would say that Buddha's qualities are not within us and these qualities can be obtained only through the accumulation of merit (good karma).
This is not true. Theravadra practices do not aim merely for the accumulation of merit, but for the realisation of the enlightenment of an Arhat (the destruction of the five poisons) as well, via the accumulation of wisdom.
I totally agree that Hinayana practitioners need to accumulate of wisdom. Here I talked about the distinction in the views on "origin" of Buddha's qualities.
According to Hinayana teachings Arhats don't differ from Buddha (who is regarded as Maha Arhat) in accumulation of wisdom (realization). That is for both there is nothing more to learn. As for the qualities of Buddha, the qualities that Arhats don't have, it is said in Hinayana teachings that these qualities are the result of Buddha's great accumulation of merit, which he practiced for 3 immeasurable kalpas (time cycles).
According to Mahayana teachings Arhats do differ from Buddha in accumulation of wisdom (realization). While Arhats eliminate clinging to the self, Arhats is not free from the clinging to phenomena. These two types of clinging are well-known two obscurations. So while for Buddha there is nothing more to learn, Arhats have some. And they do learn that after Buddha wakes them up by Buddha's light after their long rest in the peace of Nirvana, then after entering the Mahayana path they accumulate this great amount of merit and wisdom and eventually become Buddha.
That is just a sign that they didn't accumulate enough merit and wisdom in past lifes to gain the capacity to adopt Mahayana view in this one and they just mainly focus in their intention on their own benefit (like me in the mistake I made). It is not a voice of arrogance. Any time I practice virtue with the intention to attain any type of Nirvana and without intention to bring all beings to Mahanirvana (i.e. Buddha) I actually practice Hinayana path.
This is a view propounded by some Mahayana-ists in order to demean Shravakayana practitioners, please do not present it as an objective fact (unless of course you have some evidence to back it up).
I have no intention to demean Hinayana practitioners. Some beings accumulated more merit and wisdom than others. It is true and if I assert that I don't automatically demean all those who accumulated less. I just distinguish how things are. For example, Buddha Shakyamuni accumulated more merit and wisdom than me. This fact doesn't demean me, on the contrary I strive to attain the same.
While it can be not pleasant to hear that Mahayana view is superior to Hinayana view, it is true. In short while according to Hinayana view five skandhas (composites/aggregates) are free of any self but exists absolutely, according to Mahayana view they do not absolutely (1) exist, or (2) not exist, or (3) exist & not exist, or (4) neither exist & nor not exist as they are beyond these four extremes.
A Bodhisattva may "herd" beings towards liberation (shepherd-like bodhisattva, where the beings arrive at liberation before the Bodhisattva does), travel together with them towards liberation (helmsman/ship captain-like bodhisattva that arrives at liberation together with all beings) or the bodhisattva may attempt to reach Buddhahood as quickly as possible and then use their enlightenment to bring all beings to liberation (king-like bodhisattva).
According to this view shepherd-like bodhisattvas won't become Buddha until all beings are not totally liberated and helmsman/ship captain-like bodhisattvas won't become Buddha if all beings are not ready to be totally liberated.
According to scriptures of Mahayana king-like bodhisattvas become Buddha after 33 immeasurable kalpas of practice, helmsman/ship captain-like bodhisattvas become Buddha after 7 immeasurable kalpas of practice, and shepherd-like bodhisattvas become Buddha after 3 immeasurable kalpas of practice. Buddha Shakyamuni was of the last type as he perfected the path in 3 immeasurable kalpas. So according to scriptures any bodhisattva can become Buddha before all beings be totally liberated.
According to logic a moment of relative bodhichitta practice brings about a tremendous amount of merit, and a moment of absolute bodhichitta practice brings about a tremendous amount of wisdom. Now these shepherd-like and helmsman/ship captain-like bodhisattvas accumulate a tremendous amount every moment they practice bodhichitta and still can't accumulate enough to become Buddha because all beings haven't perfected these two accumulations. It's impossible that they gain more wisdom and not realize more. Eventually everyone who practice bodhichitta perfects two accumulations whether they intended to do it before other beings or not.
So the difference between these three types of Bodhisattvas is not in the result, it is in the cause - their intention. All of them intend to bring all beings to the realization of Buddha. However, in the choice of means to accomplish such a goal shepherd-like bodhisattvas do not focus on their own realization at all, helmsman/ship captain-like bodhisattvas focus on their own realization and realization of all beings equally, and king-like bodhisattvas focus primarily on their own realization.
Written not to argue, but to express another point of view
Totally off topic though