Who saved people before ~33AD?
(This question I address to those Christians who believe that salvation once acquired cannot be lost. I am specifically after the answer to the following question from that perspective)
I have always thought of salvation in the New Testament as of having the eternal life. In the New Testament we are told by John that we can get this eternal life by means of believing into the Only-begotten Son of God Jesus Christ:
For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life. (John 3:16)
These things have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God. (1st John 5:13)
However, when I am trying to look at the nature of the concept of salvation in the Old Testament, I get a bit confused about the concept of salvation that had place at those times.
First of all, the Old Testament people, of course, didn't have any knowledge of the existence of the Only-begotten Son of God and almost all of them died before this knowledge of Him was given to the human kind. However, it seems that some of them still were saved even though they died before Jesus' first coming:
There shall be weeping and gnashing of teeth, when ye shall see Abraham, and Isaac, and Jacob, and all the prophets, in the kingdom of God (Luke 13:28)
From this verse we can say that at least Abraham, Isaac, Jacob and all the prophets were saved even before Christ's crucifixion (I take it as an axiom that anyone who has the right to be in the kingdom of God is a saved person).
Now the question is at what point of their life were they acquiring that salvation? It seems that in the New Testament this point of time in people's lives is kind of more spelled out - the moment when people turn to Christ in their hearts while hearing the Gospel and pray to Him for the first time, but what about the Old Testament? At what moment of their life it was clear that they were also among those who would enter in the future into the kingdom of God (which means they were also among those who are saved)?