This is the 4183rd most frequent Polish word.
ciężkim
"Ciężkim" means "heavy" or "difficult" depending on the context.
Here, 'ciężkim' is an adjective in instrumental case describing the noun 'tygodniem' (week), expressing that the week was challenging or hard.
To był bardzo ciężkim tygodniem.
It was a very difficult week.
In this sentence, 'ciężkim' is an adjective in instrumental case referring to 'plecaka' (backpack), emphasizing its weight and carrying difficulty.
Tego plecaka nie mogę nosić, bo jest bardzo ciężkim.
I cannot carry this backpack because it is very heavy.
'Ciężkim' is used metaphorically here in instrumental case, illustrating a weighty, serious, or burdensome aspect of 'wiarą' (faith).