This is the 16512th most frequent Polish word.
wierząca
A female believer.
The word 'wierząca' functions here as an adjective describing Maria's religious belief.
Maria jest wierząca i regularnie chodzi do kościoła.
Maria is a believer and regularly goes to church.
In this sentence, 'wierząca' is used as an attributive adjective modifying 'kobieta' (woman) to signify her characteristic.
Wierząca kobieta zdecydowała się pomóc potrzebującym.
A believing woman decided to help those in need.
Here, the word 'wierzących' is a declensional form related to 'wierząca' and refers to a group of believing individuals.