The Holy Spirit's work is to grant assurance to the believer - John MacArthur

"The worst thing you can do to somebody is talk to them about Christ, and when they've prayed a prayer and verbally invited Christ in their life, then to sit there and assure them they're really saved, because you don't have any idea whether they are. That's the Holy Spirit's work; He's the one who grants assurance, and He grants it by the inward testimony, Romans 8, and by the outer exhibit of works that prove it, because faith without works is what? It's dead." 

- John MacArthur

Grace to You Radio Podcast, "Examine Yourselves Whether You Be in the Faith," Part 1.