When I began to edit your question for better formatting, I noticed that there were lemmas hidden from view because they were wrapped in
<>. Oddly, these lemmas were not even visible in the edit history. I was only able to see them when I actually clicked edit:1
Whatever Bible version you are using, it is not displaying the actual Greek text as found in a manuscript, but rather, an interlinear featuring the Greek lemmas, or lexical words. (These have relatively limited use for studying biblical Greek.)
For example, the actual Greek text of 1 Thessalonians 4:14 states,
εἰ γὰρ πιστεύομεν ὅτι Ἰησοῦς ἀπέθανεν καὶ ἀνέστη οὕτως καὶ ὁ θεὸς τοὺς κοιμηθέντας διὰ τοῦ Ἰησοῦ ἄξει σὺν αὐτῷ
On the other hand, an interlinear displaying only lemmas would read:
γάρ εἰ πιστεύω ὅτι Ἰησοῦς ἀποθνῄσκω καί ἀνίστημι οὕτω(ς) καί κοιμάω διά Ἰησοῦς ἄγω θεός ἄγω σύν αὐτός
In 1 Thessalonians 4:14, the word πιστεύομεν (pisteuomen) is the actual Greek word that is (properly) translated into English as “we believe.” The Greek text does not have πιστεύω (pisteuō), which is the Greek word you originally cited. However, even πιστεύω does not mean “they believe”; it means “I believe.” “They believe” would be translated from πιστεύουσιν.
1 I rolled back to the original post so others could view the original post without the subsequent editing.