Greek "pisteuo" "uπιστευων" (from John Salza)

Protestant:

John 3:16 "For God so loved the world that he gave his one and only Son, that
whoever believes in him shall not perish but have eternal life."

Justification, therefore, is a one-time event. Once saved, always saved.

Catholic:

To show that the same John 3:16 clearly states that justification is an
ongoing process, let's go to the Greek: Οὕτω γὰρ ἠγάπησεν ὁ Θεὸς τὸν κόσμον,
ὥστε τὸν υἱὸν αὐτοῦ τὸν μονογενῆ ἔδωκεν, ἵνα πᾶς ὁ πιστεύων εἰς
αὐτὸν μὴ ἀπόληται ἀλλ’ ἔχῃ ζωὴν αἰώνιον.

The word “believes” is “pisteuo” "πιστευων" in Greek which
necessarily includes obedience throughout one’s life. This is proved by 1 Peter
2:7-8 which also uses “pisteuo” (to obey) and “apitheo” (to disobey).

The same word “pisteuo” is used in many other verses about “believing in Christ”
such as John 3:36; 5:24; Rom. 4:24; 10:9-10; cf. Rom. 1:5,16; 6:17; 16:26; 1
John 5:13 (often used by Protestants to support their “faith alone” theology).

To “believe” means to “obey” throughout one’s life; it is not a one-time acceptance of Jesus as Savior.