Heb 10:39 But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
You could not possibly "draw back" from a work that Christ completed in you and yet there's the warning to not draw back to perdition. There it is, right in the same portion of Scripture you expound upon and yet it clearly contradicts Luther's theory of instant purification. So, some people believe Luther but I believe Scripture.
Regards
Hebrews 10:38 Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him. 39 But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul.
If a person stops having that faith in Jesus of course there will be a drawing back into perdition. Just because you lost faith doesnt mean that the completeness of Jesus sacrifice is still valid.