We believe in the eternal security of the saints. First, because they are Christ’s, and He will never lose the sheep which He has bought with His blood and received of His Father.
Next, because He gives them eternal life, and if it be eternal, well then, it is eternal, and there can be no end to hell, and heaven, and God. If spiritual life can die out, it is manifestly not eternal life, and that effectually shuts out the possibility of an end.
Observe, further, that the Lord expressly says, “They shall never perish.” As long as words have a meaning, this secures believers from perishing. The most obstinate unbelief cannot force this meaning out of this sentence.
Then, to make the matter complete, He declares that His people are in His hand, and He defies all their enemies to pluck them out of it. Surely it is a thing impossible even for the fiend of hell. We must be safe in the grasp of an almighty Savior. Be it ours to dismiss carnal fear as well as carnal confidence and rest peacefully in the hollow of the Redeemer’s hand.
[from Faith’s Checkbook by Charles H. Spurgeon]