Thanks for the answers. I think part of your confusion is that you are making no distinction between the Old and New Testaments.
In reply to:

John 14:16, yes the Spirit was given to the followers of Christ to abide with them forever. But this promise is also conditional. Remember King Saul, who grieved God so much that the Holy Spirit departed from him (1 Samuel 16:14).

The Holy Spirit could be taken from a person in the Old Testament, but that is not possible now. You are right that Saul did have the Holy Spirit leave him. In the OT, the Holy Spirit could come to someone in a temporary way. Balaam was a false prophet, yet spoke by the Holy Spirit. All accounts of "believing" in the New Testament are not the same as accounts of saving faith. Even devils believe and tremble. That may be a subject for another thread.

You are right to discern that not everyone who claims to be a Christian is one. You are wrong in your belief that a person, once washed in the blood of the Lamb and made a new creature in Christ , who has been transformed from the Kingdom of Darkness to the Kingdom of Light could ever be lost.
2 Corinthians 5:17 Therefore, if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.
Many people do fall away, but if they had been true believers, they could not.
1 John 5:13 says "I write these things to you who believe in the name of the Son of God so that you may[color:red] know that you have eternal life."

And from John 17:6 "I have manifested Your name to the men [color:red]whom You have given Me out of the world. They were Yours, You gave them to Me, and they have kept Your word. 7 Now they have known that all things which You have given Me are from You. 8 For I have given to them the words which You have given Me; and they have received them, and have known surely that I came forth from You; and they have believed that You sent Me. 9[color:red] I pray for them. I do not pray for the world but for those whom You have given Me, for they are Yours. 10 And all Mine are Yours, and Yours are Mine, and I am glorified in them.

11 Now I am no longer in the world, but these are in the world, and I come to You. Holy Father, keep through Your name those whom You have given Me, that they may be one as We are. 12 While I was with them in the world, I kept them in Your name.[color:red] Those whom You gave Me I have kept; and none of them is lost except the son of perdition, that the Scripture might be fulfilled.

2 Timothy 1:12 says "12 For this reason I also suffer these things; nevertheless I am not ashamed, for I know whom I have believed and am persuaded that He is able to keep what I have committed to Him until that Day."
As Laz pointed out there are[color:red] two categories, the sheep and the goats. There is not a third category--the goats who used to be sheep. It doesn't make sense for God to transform a person into the Kingdom of Light and then for them to be lost again into the Kingdom of Darkness because they have fallen away. Wouldn't that make Jesus a liar in his High Priestly prayer?
In reply to:

You wrote:
"He has the power to keep us safe to the end."

I agree. But I believe that we must continue to abide in Christ for Him to keep us. The scripture says that we are kept by the power of God through faith (1 Peter 1:5), but that faith can be made "shipwreck" (1 Timothy 1:19).

I agree with you, but I would say that it doesn't depend on our ability to abide. We will abide because He is holding onto us, not the other way around. We will want to abide because we are His own. I disagree that the faith that was made shipwreck was a true saving faith.