Eternal Security is an interesting topic and of course it must start with salvation. I think most believers agree on what true salvation is but many disagree on the security of the believer. If we look at how Jesus viewed salvation, and we can because we have the Gospels, we have salvation from God's viewpoint - which is a far more advantageous place to view things from. Look at what Jesus said;
“All that the Father gives Me will come to Me, and the one who comes to Me I will by no means cast out. For I have come down from heaven, not to do My own will, but the will of Him who sent Me. This is the will of the Father who sent Me, that of all He has given Me I should lose nothing, but should raise it up at the last day.” John 6:37-39
Jesus had known from the beginning which of them did not believe and who would betray him. He went on to say, “ But there are some of you who do not believe.” For Jesus knew from the beginning who they were who did not believe, and who would betray Him. And He said, “Therefore I have said to you that no one can come to Me unless it has been granted to him by My Father.” John 6:64-65
So from the divine viewpoint, we see that believers are actually a gift from the Father, to the Son. It is the Father that enables people to come to Jesus to be saved. And not only that, but of those that the Father gives Jesus, He has specifically said that He will lose none, not one. Jesus will not lose any believers that the Father gives Him. He will lose none, but will raise them all up on the last day.

Ok, some people will say we cannot be snatched out of his hand but can we decide to leave? Firstly, would a truly born-again believer ever want to lose his salvation? And secondly, if for some strange reason a person did want 'to leave', where could they go to get away from the one who now indwells them until the day of redemption? I personally do not believe that a true born again believer would ever 'want to leave' or lose their salvation. The fruit of a true salvation is shown by the fact that they do persevere to the end. When some who followed Jesus began to leave because they thought His teaching was too hard, Jesus asked the apostles if they would leave also. Peter answered this way, “Lord, to whom shall we go? You have the words of eternal life. Also we have come to believe and know that You are the Christ, the Son of the living God.” John 6:68-69
What believer who realized his need for a Savior, and you must realize that need, would walk away? This does not mean that there are not times of doubt, or even backsliding in the life of a true believer, but God does not stop working in the lives of such people and the Holy Spirit does not leave them. I welcome your comments.