Hebrews 3:14 - For we have become partakers of Christ (demonstrative evidence) if we hold the beginning of our confidence steadfast to the end.
Yes, we know that all believers are holding on to confidence in Christ. That's the definition of a believer. All believers are partakers of Christ and are holding on to confidence in Christ.
But how does this <
have> to mean that if you stop having confidence in Christ that means that you were never a partaker of Christ?
You're reading the passage through the presupposition of once saved always doctrine, and that keeps you from being able to see the passage any other way than you do.
All the passage is saying is a person who's holding onto confidence in Christ was made a partaker of Christ (and does not need to be made a partaker again). What you're not seeing is, the holding of confidence is in the Subjunctive mood which means it may or may not continue. And yet the verse is saying this holding of confidence,
that may or may not continue, is the holding on to confidence of someone made a partaker of Christ. But you say the verse is saying being a partaker of Christ means the action of having had confidence in Christ will continue and not end, because you're a partaker of Christ. The Aorist, Subjunctive mood verb 'hold'
does not allow that interpretation. You've interpreted it incorrectly and the passage doesn't support at all what you are sure it does.