heilig
holy / sacred
Regarded in a religious sense as divine, consecrated, or worthy of veneration.
Der Ganges ist für Hindus ein heiliger Fluss.
The Ganges is a sacred river for Hindus.

holy / sacred
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holy / sacred
Regarded in a religious sense as divine, consecrated, or worthy of veneration.
Der Ganges ist für Hindus ein heiliger Fluss.
The Ganges is a sacred river for Hindus.

'Heilig' (holy/sacred) is related to 'das Heil' (well-being, salvation) and the verb 'heilen' (to heal). The basic idea is something whole, unharmed, and worthy of veneration.
Die Pilger suchten an diesem Ort ihr Seelenheil.
The pilgrims sought their spiritual salvation at this place.
In the figurative meaning 'untouchable/sacred', a dative construction is often used. The structure is: [the thing] ist [the person in the dative] heilig.
Die Sonntagsruhe ist meinen Nachbarn heilig.
Sunday rest is sacred to my neighbors.
The expression 'hoch und heilig versprechen' (to solemnly promise) is a fixed idiom. 'Hoch und heilig' functions as a unit here and strongly intensifies the verb 'versprechen' (to promise).
Er hat mir hoch und heilig versprochen, pünktlich zu sein.
He solemnly promised me to be on time.
In a religious context, there are many fixed combinations with 'heilig' (holy/sacred). Well-known examples are 'die Heilige Schrift' (the Holy Scripture; Bible, Quran, etc.), 'ein heiliger Ort' (a holy place), or 'der Heilige Abend' (Christmas Eve, December 24).
Viele Gläubige besuchen die heiligen Stätten in Jerusalem.
Many believers visit the holy sites in Jerusalem.
religious
'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'.

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