Someone asked a great question of me recently. They wrote:
Is John 6:47-51 saying that, verse 49, the fathers who ate the manna would not see eternal life?”
GREAT question! Our forefathers ate manna - the food YHWH provided to sustain them here on earth. That’s not to say, they DIDN’T have “eternal” life, because they surely did, for they ALL “believed” in YHWH ELOHIM whose miracles they witnessed first-hand for 40 years. (Those who rebelled against YHWH and His chosen prophets and leaders, such as Korach and others [Numbers 16:1 through 18:32] obviously did not receive eternal life.) YHWH is, after all, the giver and taker of life.
The thing is, in the wilderness, the “bread” our forefathers ate could not save anyone from death. That “bread” was merely daily food to sustain them. It was never meant for eternal life. But ultimately, after YHWH’s people entered the Land, and as people kept becoming more disobedient and doing their own thing, and the Roman government got in the way, He sent The Word - Messiah Y'shua (John 1:1, 14) - to earth to teach His people face-to-face concerning YHWH and the Kingdom; thus giving a chance at eternal life to ANYONE who believed in YHWH and His Son who martyred himself for us.
Remember, YHWH has always required a sacrifice for sin...And, contrary to popular belief, "prayer and repentance" are NOT all that's required to enter the Kingdom.
Leviticus 17: 11 "For the life of the flesh is in the blood, and I have given it to you on the altar to make atonement for our souls; for it is the blood by reason of the life that makes atonement."
If nothing more than prayer and repentance were required, then Adam and Eve could simply have repented and mankind wouldn't be living in fallen world right now! A sacrifice was always required, which is why we are told in Genesis 3:21 that it was YHWH Himself who made the sacrifice of an innocent animal to clothe their nakedness.
The answer to your question about John 6:49 can actually be found a few verses earlier, beginning in verse 31:
John 6:31. Our fathers ate manna in the wilderness as it is written that He gave them bread from heaven to eat.” 32. Y’shua said to them, Amen, amen I say to you that it was not Moshe that gave bread to you from heaven, but my Father gives true bread to you from heaven. 33. For His bread is that of Elohim, He who has come down from heaven and gives life to the world. 34. They said to him, “Our Master give this bread to us at all times!” 35. Y’shua said to them, I am the Bread of Life. Whoever comes to me will not hunger. And whoever believes in me will not ever thirst.
36. But I have said to you that you have seen me, and you do not believe. 37. All who my Father has given to me will come, and whoever comes not to me I will cast outside! 38. For I came down from heaven, not to do my will, but to do the will of Him who sent me. 39. And this is His will, that of Him who sent me, that all that He has given to me I do not lose even one. 40. For this is the will of my Father, that all who see the Son and believe in Him, will have life that is eternal. And I will raise him up on the last day. (AENT)
 Psalm 78:24; Nehemiah 9:15.
 As in the context of Miltha (the Word) or the “Manifestation” or the “Will of” YHWH coming down from heaven to do the Will of YHWH upon earth; his Spirit is of YHWH but his nephesh (soul) and body are human and temporal.
 Y’shua reiterates that eternal life begins after the resurrection, otherwise known as the “First Resurrection” (Revelation 20:5-6) or the “Resurrection of the Righteous” (Daniel 12:1-2; Luke 4:14; Acts 24:15), not at the time of a person’s death. This event will occur within a time span known as the acharit hayamim (the End of Days), which is just prior to Mashiyach’s return (compare Revelation 20 & 22).