In the major Mahayana traditions that we have today all teach, except one, that Buddhahood is possible in a single lifetime.
I don't think all but a very small minority of people realistically think they'll achieve attainment of the dharmakāya within their lifetime. Even in the case of esoteric traditions while theoretically possible it is stressed that it requires one to have already cultivated countless roots in past lives. As my guru said to me in regards to Buddhahood in this life, "You need a perfect teacher and a perfect student."
There are different names given to this concept but what seems to cover all is "this body becomes buddha" (即身成佛), i.e. becoming buddha in this body, which has been singled out by Kukai as the term summing up the essence of Shingon (see: Principle of Attaining Buddhahood with the Present Body
), but actually can be traced back to the case with the dragon princess in the Lotus Sutra
who turned into a buddha in no time.
I wouldn't take symbolism in religious literature as absolutely literal.
This idea of sudden enlightenment (頓悟) - first advocated by Daosheng (355-434), a disciple of Kumarajiva - is taken to be an improvement, a higher teaching compared to the gradual path of the bodhisattva going through kalpas of training. But it is possible to see it not as an improvement but actually a return to being an arhat.
That term means different things to different people. We must keep in mind that "enlightenment" (悟), whether it be gradual or sudden, is not equivalent to Buddhahood (成佛). You can be enlightened and not be a Buddha.
What I mean is that there's hardly any difference between the attainment taught in these sudden methods from one we can find in Theravada and other Mahayana teachings. The basic concept is, as always has been, to eliminate attachment to the six kinds of experiences. That is what an arhat achieves.
Mahayana thought the idea is that the Arhat only achieves nirvana with remainder or residue (有餘涅槃 / sopadhiśeṣa-nirvāṇa).
For quick reference let me provide the entry from the DDB:
Nirvāṇa with residue. Nirvāṇa attained while living in this world. Nirvāṇa attained while still having a body. Also called “lesser vehicle nirvāṇa,” because it can be attained by śrāvakas 聲聞 and pratyekabuddhas 緣覺. The term refers to the manifestation of true thusness coincident with the extinction of the afflictive hindrances. Abbreviated as 有餘依 and written more fully as 有餘依涅槃. Pre-Mahāyāna teaching held that the arhat after his last term of mortal existence enters into nirvāṇa, but while finishing out this life he is in the state of sopadhiśeṣa-nirvāṇa. This is a form of nirvāṇa that is contrasted with nirvāṇa with no residue 無餘涅槃 (nirupadhiśeṣa-nirvāṇa). Mahāyāna holds that when the cause 因 of reincarnation is ended the state is that of 有餘涅槃 incomplete nirvāṇa; when the effect 果 is ended, and 得佛之常身 the eternal Buddha-body has been obtained, then there is 無餘涅槃 complete nirvāṇa. Mahāyāna writers say that in the Hīnayāna 無餘涅槃 “remainderless” nirvāṇa for the arhat there are still remains of illusion, karma, and suffering, and it therefore has residue. In the Mahāyāna nirvāṇa without residue these remains of illusion, etc. are ended. One of the “four kinds of nirvāṇa” 四種涅槃 in Yogâcāra theory. 〔瑜伽論 T 1579.30.789b20, 成唯識論 T 1585.31.55b27〕 (Skt. sopadhiśeṣa-nirvāṇa) [cmuller; reference(s): Yokoi,Hirakawa,JEBD]
Cited from: Digital Dictionary of Buddhism: 有餘涅槃 | nirvāṇa with remainder http://www.buddhism-dict.net/cgi-bin/xp ... 85-69c3%27
In this theory the Arhat has really yet to achieve ultimate elimination of attachments.
Obviously a śrāvakayāna proponent could find plenty of reason to disagree with what a Mahayana advocate would propose concerning this. For example in the case of an Arhat post-mortem where are they if there are no aggregates?
One response to this is stating that the Arhat abides in the samadhi of non-abiding and that due to the remains of illusion, having not truly comprehended emptiness, they are eventually arisen from it and become Bodhisattvas.
Now let us examine the opposite term of nirvana without remainder (無餘涅槃):
# Nirvāṇa without residue. Unconditioned, unlimited nirvāṇa; the state of total liberation from all physical and mental conditions. This is in contrast to nirvāṇa with remainder 有餘涅槃, where the body still exists. Also written as 無餘依涅槃. One of the four kinds of nirvāṇa in Yogâcāra the afflictive hindrances 煩惱障 in the mind are cut off, and the body that is composed of the five aggregates is extinguished. Therefore there is nothing remaining to depend upon. In this nirvāṇa, all afflictive hindrances are destroyed, so it can be attained by śrāvakas and pratyekabuddhas. (Skt. anupadhi-śeṣa-nirvāṇa, nirupadhiśeṣa-nirvāṇa, nirupadhiśeṣa, nirupādi-gati; Pali anupādi-sesa-nibbāna; Tib. lhag med myang 'das) 〔瑜伽論 T 1579.30.375c21, 成唯識論 T 1585.31.13c7, 起信論 T 1666.32.581a2〕 [cmuller; source(s): Nakamura,YBh-Ind,Hirakawa]
Cited from: Digital Dictionary of Buddhism: 無餘涅槃 | nirvāṇa without remainder http://www.buddhism-dict.net/cgi-bin/xp ... 85-69c3%27
Then compared to an arhat a buddha is supposed to have some extras starting with the bodily signs, so comes in the accumulation of merits through aeons. Finally, with the appearance of the tathagatagarbha teachings the possibility of a short cut came up, since buddha-nature contains all the buddha qualities. That's how sudden enlightenment became possible, that's what the Zen axiom of "this mind is buddha" (即心是佛) stands for.
Tathāgatagarbha is not single unified system. In general there seems to be two ideas about Buddha nature. The first, which is found in the Treatise on Buddha Nature
(佛性論), in no uncertain terms states that Buddha nature is one of three types of potential causal-processes for beings to attain the dharmakāya
For your reference please examine the following diagram I drew:
In this model Arhats are subsumed under the prāpya-hetu
However, in this model the three causal types are not mutually exclusive. One can initiate the cause towards Buddhahood through understanding emptiness, aspiration or through the appropriate practices, but we can also say that these causes mutually support one another and in reality they do just looking at prāpya-hetu
This model does not suggest one can instantly attain the dharmakāya. However, later thinkers and others, whom I think were more often than not speaking figuratively, proposed one could achieve the dharmakāya, which is synonymous with Buddhahood, in a lifetime. Later on the time frame was removed and we have statements such as, "This very mind is
Buddha!" thus negating that you ever really achieve Buddhahood because right this instant you are
Buddha. However, such statements, while valid in context, are often distorted and misconstrued. When you become a Buddha linear time becomes unreal and thus right now, as well as all past and future moments, become encompassed within your awareness. However, until you get to such a point it is probably best to refrain from actively saying or thinking that your mind and very being is Buddha.
Then if there is this inherent enlightenment (本覺), which is not different from the mind free from attachments, how could it be different from an arhat but in name only?
The idea of original or inherent enlightenment (本覺) is a product of East Asian thought and has no direct ancestor in Indic thought. To my understanding the first appearance of this term is in the Awakening of Faith in the Mahayana
大乘起信論, which while attributed to Aśvaghoṣa was most likely composed in China. This being said I don't see how you can relate this term to Arhatship which by the time said text appears was a pejorative in Chinese Buddhism.
However, for the sake of discussion not limited in academic understanding, let me say this: one simple way of understanding this is that Arhat has not achieved complete elimination of attachments ergo they are not ultimately enlightened which in this case means attainment of the dharmakāya.
Again, the key point here is that in the context I have outlined above "enlightenment" in the sense of "Buddhahood" is specifically defined as attainment of the dharmakāya. In English these terms are more often than not conflated and carelessly tossed around without proper definition first.
In this sense the Arhat does not even seek attainment of the dharmakāya and within their system of dharma they have no means to even work towards it. The goal of the śrāvakayāna is the cessation of rebirth and their model works quite well for that aim. However, to explain why they do not achieve full realization of tathātā and by extension the dharmakāya despite having purportedly cut off defilements and cravings, it is said by some Mahayana thinkers that they have not actually eliminated all their defilements particularly at a subtle level. It is for this reason that Arhats are said to attain a nirvana with remainder or residue even after their supposed final rebirth. In said model they will eventually voluntarily take rebirth which would require the appropriate career as a Bodhisattva which precedes Buddhahood.
What this discussion boils down to is how you define the term "enlightened" or "realized". The Arhat is indeed enlightened, but her enlightenment is not Buddhahood. The Bodhisattva likewise is enlightened, but she has yet to achieve the dharmakāya or in other words Buddhahood.