THis is something I've always had lots of questions about myself. Of course, I agree that OT and NT believers are saved in the same manner, but as you point out it's often difficult to put the reason for believing this into words. You might want to read through this thread

https://www.the-highway.com/forum/showthr...prev=#Post43379

I'm not sure it will answer your question, but I think the Warfield article that Pilgrim links to in the thread contains some good information about the work of the Holy Spirit in both NT and OT saints.

John