The LORD knows that He could give eternal life to believers even before the Cross because He knew that He would redeem the sins of believers who lived before the Cross:
"And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first covenant, they which are called might receive the promise of eternal inheritance. For where a testament is, there must also of necessity be the death of the testator. For a testament is of force after men are dead: otherwise it is of no strength at all while the testator liveth" (Heb.9:15-17).
Do you think that God lacks the power to give eternal life to believers in every age, even before the Cross, knowing that their sins would be redeemed at the Cross?
In fact, before the Cross the sins of believers were not even imputed to them,as witnessed by what Paul said about David here:
"But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness. Even as David also describeth the blessedness of the man, unto whom God imputeth righteousness without works, Saying, Blessed are they whose iniquities are forgiven, and whose sins are covered. Blessed is the man to whom the Lord will not impute sin" (Ro.4:5-8).
We also see Peter saying the following:
"To him give all the prophets witness, that through his name whosoever believeth in him shall receive remission of sins" (Acts 10:43).
Before the Cross the Lord Jesus said the following to a Jewish woman:
"And he said unto her, Thy sins are forgiven. And they that sat at meat with him began to say within themselves, Who is this that forgiveth sins also? And he said to the woman, Thy faith hath saved thee; go in peace" (Lk.7:48-50).
With these things in mind there is absolutely no reason why God couldn't give believers eternal life before the Cross. And we know that before the Cross believers did receive eternal life when they believed, as witnessed by the following words of the Lord and Savior:
"Very truly I tell you, the one who believes has eternal life" (Jn.6:47).
You still have not given an interpretation of the meaning of this verse which makes any sense.