...and the stipulation was believes, which by definition in the plural requires a continuation for it to be possessed or Christ could have said, the one who believed because belief has to be past tense before possession in its truest form because Christ is speaking to us in a singular form when he says one... So, one who believes requires continuation.
If it took continuing in faith then the Lord would not have told the Jews who lived under the law that they possessed eternal life when they believed:
"Very truly I tell you, the one who believes has eternal life" (Jn.6:47).
The word "believe" in these verses is in the "present" tense and is therefore "durative".
According to the Greek experts the Greek present tense can be in regard to a continuous action beginning in the past and continuing into the present time:
"The durative (linear or progressive) in the present stem: the action is represented as durative (in progress) and either as timeless or as taking place in present time (including, of course, duration on one side or the other of the present moment" (Blass & DeBrunner, A Greek Grammar of the New Testament and Other Early Christian Literature, p. 166.)
"The present tense may be used to describe an action which, begun in the past, continues in the present. The emphasis is on the present time" (Wallace, Greek Grammar Beyond the Basics, p. 519).
"A Present Tense form is called durative when the context conveys an action that began in the past and continues into the present" (Young, Intermediate Greek, p.111-112).
The Lord was not telling anyone that they must continue to hear Him speaking in the distant future in order to be saved. Instead, those who were believing Him as He spoke His words were at that moment passed from death unto life and will not come into judgment:
"Very truly I tell you, whoever hears my word and believes him who sent me has eternal life and will not be judged but has crossed over from death to life" (Jn.5:24).
You still did not specifically address the meaning of these words...."otherwise it is of no strength at all while the testator liveth"..Heb.9.15-17. What do they mean?
They mean exactly what they say. Without the Cross no one could be saved from eternal destruction. However, since the Father knew that the Lord Jesus' death on the Cross would redeem the sins of the believers who lived before the Cross (Heb.9:15) He remitted those previous sins through His "forbearance":
"Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God" (Ro.3:25).
In other words, God tolerated the sins of believers before the Cross and even remitted their sins knowing that at the Cross the Lord Jesus's blood would redeem the sins of all the believers who lived before the Cross. Therefore, since their sins were taken away before the Cross the Lord Jesus told believers that they already possessed eternal life.
However, you seem to think that God is powerless to do these things even though the Scriptures states that with God all things are possible.