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Well, check it out under Bible Hub, and then also consider verses which DO say their faith (which is phrased like "the faith OF THEM" ['of them' is a different Greek word]... though we say it like "their faith". In this verse, it is the definite article "the".)
Perhaps it will also help you to look at another word, along with this:
where 1Tim1:19b uses the word that means "to thrust away, reject, refuse, push away, repudiate, to drive away from oneself, having cast away" (see who does this; are either of these talking about people who once were "saved" [eternal life] and then cast off that salvation [eternal life]? or were they in possession of truths which they thrust away/rejected/drove away from oneself [disregarded as not for them]?) -
https://biblehub.com/text/acts/7-39.htm
https://biblehub.com/text/acts/13-46.htm
[see the wider contexts]
Perhaps it will also help you to look at another word, along with this:
where 1Tim1:19b uses the word that means "to thrust away, reject, refuse, push away, repudiate, to drive away from oneself, having cast away" (see who does this; are either of these talking about people who once were "saved" [eternal life] and then cast off that salvation [eternal life]? or were they in possession of truths which they thrust away/rejected/drove away from oneself [disregarded as not for them]?) -
https://biblehub.com/text/acts/7-39.htm
https://biblehub.com/text/acts/13-46.htm
[see the wider contexts]
You can't stay in Christ and reject the word of the gospel. It's that simple.