Jesus said that because the Jews in Israel were conditioned by the Pharisees to seek salvation by works. When a new rabbi leader arose the Jews sought him out to see what works he would prescribe to gain eternal life. That is why we find passages like the following...
"Now a certain ruler asked Him, saying, “Good Teacher, what must l I do to inherit eternal life?” Luke 18:18
"And behold, a certain lawyer stood up and tested Him, saying, “Teacher, what shall I do to
inherit eternal life?” Luke 10:25
Jesus wanted to weed out the 'works for salvation' mentality that had been following Him.
That is why, knowing full well how they would react, he said what he did in John 6:53-55.
Jesus said to them, “Most assuredly, I say to you, unless you eat the flesh of the Son of Man
and drink His blood, you have no life in you. Whoever eats My flesh and drinks My blood
has eternal life, and I will raise him up at the last day. For My flesh is food indeed, and My
blood is drink indeed." Jn 6:53-55
The Jews seeking for some works to do took Jesus literally. Jesus was speaking in a spiritual sense.
Jesus did that to get them to stop following and taking up space by those who he knew would not be productive.
Yet there are some of you who do not believe.” For Jesus had known from the beginning
which of them did not believe and who would betray him. He went on to say, “This is why I
told you that no one can come to me unless the Father has enabled them.”
From this time many of his disciples turned back and no longer followed him. John 6:64-66.
Its not by works (plurality of religious prescribed deeds to perform) that we are saved.
Its only by God's drawing of a soul that will bring a person to the point of faith in Christ that saves.
Jesus said that there is only one work for salvation the Father approves of.
Not by works (religious system). But, by one work alone!
Then they asked him, “What must we do to do the works (plurality) God requires?”
Jesus answered, “The work (singular) of God is this: to believe in the one he has sent.” jn 6:28-29
grace and peace ...................