Does Once Saved Mean Always Saved?

Psalm 3:8 says Salvation belongs to the Lord

A commentary from The Treasury of David by Charles Spurgeon:

"This verse contains the sum and substance of Calvinistic doctrine. Search Scripture through, and you must, if you read it with a candid mind, be persuaded that the doctrine of salvation by grace alone is the great doctrine of the word of God: "Salvation belongeth unto the Lord." This is a point concerning which we are daily fighting. Our opponents say, "Salvation belongeth to the free will of man; if not to man's merit, yet at least to man's will;" but we hold and teach that salvation from first to last, in every iota of it, belongs to the Most High God. It is God that chooses his people. He calls them by his grace; he quickens them by his Spirit, and keeps them by his power. It is not of man, neither by man; "not of him that willeth, nor of him that runneth, but of God that showeth mercy."