This is how.
If you remain in His word, you ARE a true disciple. So what He is saying here, is true disciples remain.
It's like in Hebrews, I forget the reference, You have become a partaker of Christ if you continue firm in your faith to the end. Same word "firm" there, not certain of your faith, but unmoved in your faith. The part I want to highlight is the Perfect Tense of 'have become', showing this is something that happens and remains done.
You have become a partaker of Christ, a once for all thing, if you continue in your faith. If you do not continue in your faith, then you did not become a partaker of Christ. The "continuance of faith" is not what determines whether you will partake of Christ, rather, it shows whether you did or did not actually become a partaker in Christ.
Much love!