This is regarding "a grown matured adult" who already knows the Religion and its requirements (obligations, etc.) and knows very well the outcome of not following and practicing it. Basically, it is with regards to prayer. A Muslim male adult who, despite knowing the obligations of his religion and the punishments, still fails to perform daily obligatory prayers. (Ironically his faith and beliefs in Allah, are more evident than that of those who do pray!).
How is one to talk to this kind of person to start praying and practicing as its a loss for him.

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

A pubescent Muslim is required to carry out all religious duties and he is held accountable by Allah for any act that he performs. Therefore one has to remind this person of Allah, of death and its agonies, the grave and its punishment, and of the Day of reckoning, which is a dreadful Day. What would be his situation if he meets Allah while not performing prayers?

Some of the people of knowledge are of the view that whoever does not perform prayers is Kafir (non-Muslim) and is out of the fold of Islam. So, would this person agree to die as a Kafir?

Advise him to attend lectures on religious knowledge, to visit the Houses of Allah (mosques) and to keep company with pious and righteous people only. This will help him, Insha Allah, to be steadfast on the religion of Allah. Al-Bukhari and Muslim reported that Ibn Mas'oud, may Allah be pleased with him, narrated that the Prophet said: "A person is with whomever he loves." Please refer to the following fatawa: 81555, 82076, 82961, 83565 and 83943 for more details.

Allah knows best.