This is the 886th most frequent Japanese word.
信じ
"Belief" or "to believe."
The word "信じ" (shinji) is used as the verb "to believe" in the present/future tense, expressing trust in someone's words.
私はあなたの言葉を信じます。
I believe in your words.
Here, "信じ" (shinji) is used in the progressive form "信じています," showing an ongoing belief in Santa Claus.
子供たちはサンタクロースを信じています。
Children believe in Santa Claus.
In this instance, "信じ" (shinji) is in its base form used with "べきではない" to convey that believing his story is not advisable.