Hi Pickles,
Job's wife told him to curse God and die. It was not in Job's heart to do so. But we know from what John wrote that there were some who had gone out from them, from the body. But John says that they never were really of the body, else they would not have left.
I believe that some profess Jesus, and as He said, they are promised salvation, if they remain steadfast in their confession. But some confess, and never come to the knowledge of Jesus. That is, they never come to know Him, never enter into the life saving relationship with Him that is life, that is salvation. And from there we are called to go farther, and expand this relationship to include the Father.
No one can take one from His hand who hears His voice and is known by Him. Notice how in His promise that He goes on and brings His Father into it, saying that no thing is greater than the Father to take out of His hand? He finishes by saying that He and the Father are one.
Blessings sister