This word ‘religious’ is more commonly used as an adjective, and as a noun it has become very rare:

1. He is religious. = He believes in God and religion, but continues to live a normal life. (He is not an atheist.) — Adjective

2. He is a religious person. = He believes in God and religion, but continues to live a normal life — gets married, has children, get a job, etc.. (He is not an atheist.) — Adjective

3. He became religious. = He had been an atheist before and somehow changed his beliefs and began to believe in God and religion, but continues to live a normal life — got married, had children, got a job, etc.. — Adjective

However,

4. He became a religious.  = He had been an ordinary person before but some changed his mind and gave up his materialistic life and made vows to spend the rest of his life in the service of God — did not marry, did not have children, did not look for a job, etc. (He was like a monk.) — NOUN

*5. She is just religious, not a religious.  = She is an ordinary woman who believes in God and religion,  not like a nun who has given up everything — no marriage, no children, no job, etc. — to be in the service of God, living in a nunnery or convent. (Note: The first ‘religious’ is an adjective, and the second ‘religious’ with the article ‘a‘ is a noun.)

6. They became religious.  = (Note: In this context, ‘religious’ can be the plural of noun ‘religious’ or can be an adjective, if no other phrase suggests otherwise.) They had been ordinary people before but later changed their minds and gave up their materialistic lives and made vows to spend the rest of their lives in the service of God — did not marry, did not have children, did not look for job or did not earn money, etc. (They were like monks or nuns.)