
gläubig
devout
A person who has a firm religious faith, especially in a god or the teachings of a religion.
Sie ist eine tief gläubige Christin und geht jeden Sonntag in die Kirche.
She is a deeply devout Christian and goes to church every Sunday.
devout / credulous
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devout
A person who has a firm religious faith, especially in a god or the teachings of a religion.
Sie ist eine tief gläubige Christin und geht jeden Sonntag in die Kirche.
She is a deeply devout Christian and goes to church every Sunday.

credulous
Trusting someone or something in a naive or uncritical way. Often used slightly ironically or in reference to children's innocence.
Die Kinder lauschten den Märchen der Erzählerin mit gläubigen Augen.
The children listened to the storyteller's fairy tales with credulous eyes.
The adjective 'gläubig' (devout/credulous) is derived directly from the noun 'der Glaube' (faith/belief). It describes a person who has faith or belief.
Er ist nicht religiös, aber er hat einen festen Glauben an das Gute im Menschen.
He is not religious, but he has a firm belief in the good in people.
You can tell from context whether the religious or the naive meaning is intended. If it is about religion, God, or church, it is the first meaning. In all other cases, often in connection with children or stories, the second, uncritical meaning is intended.
Mein Großvater war ein sehr gläubiger Mann.
My grandfather was a very devout man.
To describe the intensity of religious faith, 'gläubig' (devout) is often combined with adverbs like 'tief' (deeply), 'fest' (firmly), or 'streng' (strictly).
Sie kommt aus einer streng gläubigen Familie.
She comes from a strictly devout family.
In the meaning 'naively trusting', 'gläubig' (credulous) is often used to describe a facial expression. Typical combinations are 'gläubige Augen' (credulous eyes) or 'ein gläubiger Blick' (a credulous look).
Mit gläubigen Augen blickte der Junge zum Zauberer auf der Bühne.
With credulous eyes, the boy looked up at the magician on the stage.
Christian
For people, christlich can indicate an affiliation with or orientation toward Christianity without saying anything certain about their personal faith. Gläubig, by contrast, means that the person genuinely has a firm religious belief. Gläubig is therefore more precise for a personal profession of faith.

Nora bezeichnet sich als gläubig, weil ihr persönlicher Glaube ihr sehr wichtig ist.
Nora describes herself as devout because her personal faith is very important to her.
Nora bezeichnet sich wegen ihrer kulturellen Herkunft als christlich, obwohl sie nicht persönlich glaubt.
Nora describes herself as Christian because of her cultural background, even though she does not personally believe.