It is the position of Reformed theology, as well as some Arminian denominations, that one who has been given a true, saving faith by God cannot be lost.
Here are a few essays:
Can a Christian Lose His or Her Salvation? by Greg Johnson
Assurance of Salvation: Can I Really Be Sure? John G. Reisinger
The Blessing of Full Assurance May 13th, 1888 by C. H. SPURGEON
On one of my "favorite Christian women" bloggers, Carrie "Of Christian Women", started a series based on one of John MacArthur's sermon series. She was challenged with a series of "difficult" passsages.
I'm going through them here because it made for a rather long post.