religiös
religious
Relates to belief in one or more deities and the practice of a religion.
Ihre Familie ist sehr religiös und feiert alle kirchlichen Feste.
Her family is very religious and celebrates all church holidays.

religious
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religious
Relates to belief in one or more deities and the practice of a religion.
Ihre Familie ist sehr religiös und feiert alle kirchlichen Feste.
Her family is very religious and celebrates all church holidays.

The adjective 'religiös' (religious) is derived directly from the noun 'die Religion' (religion). It describes everything that has to do with religion or is shaped by it.
In der Schule lernen wir über verschiedene religiöse Traditionen.
At school, we learn about different religious traditions.
If 'religiös' (religious) describes a person, it means that this person is devout or has faith. If it describes a thing (e.g. a book, a building, a topic), it means that the thing has a connection to religion.
Meine Tante ist eine sehr religiöse Frau.
My aunt is a very religious woman.
In the figurative sense, 'religiös' (religious) describes an action that is carried out with extreme, almost ritual care. In this use, it has nothing to do with faith and is often intensified by adverbs like 'fast schon' (almost) or 'geradezu' (downright).
Er putzt sein Auto jeden Samstag mit einer fast schon religiösen Hingabe.
He cleans his car every Saturday with almost religious devotion.
holy / sacred
'heilig' and 'religiös' both have to do with religion, but they do not mean the same thing. 'heilig' describes something that is especially venerated in a religion or regarded as divine, for example a place, a book, or a day. 'religiös' describes people, actions, topics, or things that have to do with religion. Learners may confuse the words because both occur in the religious sphere. Memory aid: 'religiös' means 'connected with religion'; 'heilig' means 'especially venerated or consecrated'.

Im Unterricht sprechen wir heute über religiöse Feste.
In class today, we are talking about religious festivals.
Für viele Menschen ist dieser Ort heilig.
For many people, this place is sacred.