The Arhat means the enlightened one and is the highest ideal in Theravada Buddhism however The path of the Arhat is quite complex and rigorous and require strict adherence to the Eightfold Path. by following the principles advocated by the Eightfold path for example right concentration and right meditation one will begin to comprehend certain concepts of the Buddhist worldview such as Anicca, Anatta, and Dukkha on not only an intellectual level but in a non-cognitive way . One will also begin to eliminate certain elements of Tanha(Craving) known as the the Three Poisons(Greed, Hatred, and Delusion) by adhering to certain moral principles advocate by Right Action and Right Though and eventually one will attain Nibbana however all of this is done is a series of stages and takes usually multiple rebirths in order to effectively explain the Arhat Path one must do so by explaining each stage.
The stream enterer is the first stage of the Arhat Path at this stage one will have truly understood the concept that nothing in the world is permanent(Anicca) and realise that one must go beyond living a moral lifestyle in order to achieve Nibbana. At this stage one will achieve Nibbana in less than seven rebirths and will never be reborn in a lower realm however it should be understood that reaching stage is extremely difficult and one must be extremely committed to do so. the second stage is that of the once-returner at this stage one will begin to abandon feelings of ill will and attachment to sensual pleasures among other things and will achieve Nibbana within the next life. the 3rd stage is that of the non-returner this individual still has elements of delusion so therefore cannot achieve Nibbana in this life but will be reborn in the God realm which he will achieve Nibbana eventually. Finally the stage of the Arhat is when the individual has achieved Nirodha, which is the Cessation of Dukkha and of all it’s causes this individual will then enter Nibbana, so despite his physical being still remaining in Samsara he will be in an enlightened state and upon death enter Parinibbana.
In order for one to become an Arhat one must choose the Middle Path and strictly adhere to the Eightfold Path. Adhering to the Middle Path means that one must live in a way which is just as to sustain the person’s health and body because the body is the vehicle for spiritual progression and therefore should be taken care of, there no aesthetic practice and no life of excess. The Eightfold path can be seen as guide for moral living by following concepts such Right Action, Right Thought, and Right Livelihood one is maintaining a lifestyle which prevents the buildup of unwholesome Kamma and allows for the weakening of the 10 fetters which is what binds us to Samsara. By following the principles of Right Concentration, Right Effort, and Right Mindfulness one is dedicating the necessary amount time to meditation which will led to an understanding of concepts such Anicca and Dukkha. In conclusion the Arhat Path is one of extreme discipline and moral behaviour which in multiple rebirths will lead to the attainment of Nibbana.
The Laity and Monastic Sangha encompass the Four Fold Sangha, the Four Fold Sangha being the followers of the Buddha. The Monastic Sangha comprises of Bhikkhus(monks) and Bhikkhunis(nuns) who have undertaken the path of the arhat while the Laity comprises of regular people who have simply taken refuge in the Buddha, the Dhamma, and the Sangha in other word they are your average Buddhist.
The best way to summarise the relationship between the laity and the Monastic Sangha is that it is one of reciprocity, meaning they are both interdependent on each for a number reason. Seeing as the monastic way of life is one of poverty and traditionally a monk may not carry any money and his only possessions are things like a razor, an alms bowl, belt, robes, etc.. so the laity serves as the economic base for the monastic sangha, this mean they provide all the necessary items that monk would need to survive such as food, land, material needs, and volunteers from the laity take on the administrative responsibilities of the monastery so that the Bhikkhus may focus solely on spiritual progress. In exchange however the monks provide the laity with spiritual guidance in variety of ways. The first benefit which monks bring to the laity is that by donating to the monastic sangha the laity are given an opportunity to increase their merit in that their actions are Dana(giving) and Dana is conducive to wholesome Kamma, which in itself leads to the attainment of nibbana. The monastic sangha is also responsible for revealing the Dhamma to the laity and helping them on their path to enlightenment so an example of this would be when the early monastic sangha would go around villages seeking donations with their alms bowl, people would give them food or other things and they would then reveals the Buddha’s teachings to them giving them guidance on their path to enlightenment. Now however monks have resigned themselves to monasteries yet the relationship is still very similar with their adjacent communities
However as local communities started to shift away from traditional livelihoods Buddhist monks took on more practical responsibilities towards the laity, for example they would serve an educational purpose as they were often the most literate in the community and would take on the study of astrology and medicine providing the community with knowledge. They also oversaw marriages and death rite along with settling disputes within communities. For some monks the amount responsibilities towards the laity were too numerous and this led to development of forest dwelling Bhikkhus who isolate from the rest of the world spent more time focusing on the attainment of enlightenment.
Theravada Buddhists reject the three vehicles on the grounds of how one becomes enlightened. Theravada Buddhists focus on the Ahrat path, which is progressed through due to individual effort by progressing through the Jahannas. This is contrasted with the Mahayana view of Bodhicitta; the view that one obtains enlightenment by moments of great compassion for other people. The Dalai Lama calls this phenomenon 'selfish altruism' as to draw closer to enlightenment you are actively helping other people. However, this directly contrasts with the individualistic view of Theravada Buddhism causing Theravada reject the three vehicles on grounds of enlightenment.
Another area in which Theravada and Mahayana Buddhism clash is concerned with the four noble truths. Theravada Buddhists revere the Pali Canon a lot more than Mahayana Buddhists do thus meaning the four noble truths hold a lot more weight. Indeed, they have been called the 'essence of Buddhism' by Theravadins. However, Mahayana Buddhists believe the essence of Buddhism is the concept of 'dependent origination,' thus declaring everything dependent to be empty. The rejection of this concept being the essence of Buddhism is shown by the Theravdin scholar Bhikku Thanissaro who states that 'the principle of dependent origination is only an expression of The Four Noble Truths because they explain the arising of Dukkha.' This shows that The Four Noble Truths remain at the essence of Buddhism as they incorporate the same aims as dependent origination. This leads to the rejection of Mahayana Buddhism as in Theravada eyes its theory is sticks closer to the actual Pali Canon instead of other sources which do not hold the same authority The last, and arguably most realistic, reason why Theravada Buddhism rejects the three vehicles is largely due to socio-economic factors. As Theravada Buddhism really established its home in Anuradhapura in Sri Lanka by Emperor Asoka's son, the island nation saw no need to expand its influence and focus on the individualistic enlightenment factors.
However, when compared to the 'empire' of sorts originating from India with Mahayana Buddhism, which emphasises the altruistic approach, it is not hard to see how the divide between the two s,ects was furthered and thus the rejection of the three vehicles occurred .