"And I will pray the Father, and He shall give you another Comforter, that He may abide with you for ever." John 14:16
For how long? Forever. Good theology: Believe what God says before you believe what someone says about what God says. The Godhead; the Father the Son and the Holy Spirit abides with us from the moment we accept Christ as our Saviour. I know that many religions have different rules about salvation, but clear uninterpreted truth direct from scripture should not be quickly dismissed.
There are a few reasons I am making this point.
First: Jesus is praying the Father that the Father will give you the Holy Spirit. If Jesus prayed that for me, I would know for certainty that it was for all eternity. I don't believe that God the Father, the Son or the Holy Spirit makes mistakes, or qualifies someone for salvation that they have to later rescind, or change their mind about how secure salvation is for the believer. Jesus prayed the Father, the Father sent the Holy Spirit and the Holy Spirit resides within the believer.
Secondly: All that being said, everything to do with God the Father, the Son and the Holy Spirit are eternal things. Do you really think that the one most important thought of all, the salvation of believers (the joining to God of fallen man to become joint heirs with Christ by adoption) is ill conceived? Do you really believe that Christ died for your sins and when you were born again that what Christ did through the atoning sacrifice was not quite good enough to last? Are you of the persuasion that in order to be fully saved you have to jump through hoop number two or three or that being saved is just temporary as long as you don't do or say something wrong? If that were the case none of us would have a chance.
Thirdly: Salvation is a finished work of God for all eternity. Sanctification is the ongoing process of being purified in word and deed. But being sanctified makes you no more saved than you were before you were sanctified. Being sanctified makes you no more holy either. Being sanctified does make you more cognizant of your spiritual responsibilities, but standing before God there are not two lines, the saved line on the left and the sanctified line on the right. Just the thought of it reduces the possibility of the sanctified having a better place before God, because God lifts up the humble and abases the proud.
Those of you who treat salvation as a lesser gift should be ashamed. And you need to get your theology straight. Salvation is the All in All work of God that all other things spiritual come under. Sanctification, holiness, purification, spiritual gifting, fruit of the Spirit, gifts of the Spirit, measure of faith, all fall under salvation. None of these things exist unless a person is saved. And when a person is saved all of these things become part of the process of being with God forever.