According to Protestants who don't accept something like the real presence, why was Jesus' teaching at John 6 so hard?

The text seems self-explanatory:

^{John 6:58-62} This is the bread that came down from heaven, not like that which your ancestors ate, and they died. But the one who eats this bread will live forever.” He said these things while he was teaching in the synagogue at Capernaum. When many of his disciples heard it, they said, “This teaching is difficult; who can accept it?” But Jesus, being aware that his disciples were complaining about it, said to them, “Does this offend you? Then what if you were to see the Son of Man ascending to where he was before?

From context, it would appear that it is the coming down from heaven and ascending to [heaven] part that the audience was having difficulty with; this echoes other similar passages within the same Gospel, intimating Christ's connection to the divine:

^{John 10:30-33} I and my Father are one. Then the Jews took up stones again to stone him. Jesus answered them, Many good works have I showed you from my Father; for which of those works do ye stone me? The Jews answered him, saying, For a good work we stone thee not; but for blasphemy; and because that thou, being a man, makes thyself God.

^{John 10:38-39} [T]hough ye believe not me, believe the works: that ye may know, and believe, that the Father is in me, and I in him. Therefore they sought again to take him: but he escaped out of their hand.

Similar examples from the Synoptics, as well as the Book of Acts (7:55-58), could also be provided, but these shall suffice for now.

Let's start by clarifying the OP idea that "real presence" is identical to the Catholic belief in "transubstantiation". They are not the same at all.

"[transubstantiation is] the change of the whole substance of bread into the substance of the Body of Christ and of the whole substance of wine into the substance of the Blood of Christ. -source-

In Christian theology, the real presence of Christ in the Eucharist is the doctrine that Jesus is present in the Eucharist, not merely symbolically or metaphorically,1 nor literalistically, but sacramentally. -source-

So, what about John 6?

The specific idea of eating Christ's flesh and drinking Christ's blood is given to us by Christ Himself at the start of the conversation. The crowd wants the manna from heaven, but how does that work?

And Jesus said unto them, I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst. John 6:35

Coming to Christ is eating. Believing on Christ is drinking. So, then we come to verse 54 with the application.

Whoso eateth my flesh [by coming to Him], and drinketh my blood [by believing in Him], hath eternal life; and I will raise him up at the last day.

So, what is the hard saying of verse 60?

Many therefore of his disciples, when they had heard this, said, This is an hard saying; who can hear it?

Flesh begets flesh. They heard nothing spiritual, even though Christ had told them what it meant at the beginning of the discourse. Still, He tells us again.

It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. But there are some of you that believe not. For Jesus knew from the beginning who they were that believed not, and who should betray him. And he said, Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father. John 6:63-65

To conclude, the Father draws us to come to Christ, to believe His words.

Context, context, context. The invisibility of God is taught in the Scriptures (John 1:18). At John 6:46 Jesus says, "Not that any man has seen God, "Save he which is of God." The manifestation of God through the Son is the subject of Jesus' discourse, John 6:47.

Jesus refers to eternal life at vs47 and now He connects the "believeth" to himself as the nourishing, sustaining, and maintaining power of that present possession of everlasting life. At vs48 He says, "I am the bread of life." At vs49 the Jews wanted a sign like the manna in the wilderness.

At vs51, Jesus says, "I am the living bread that came down out of heaven. If anyone eats of this bread, he shall live forever; and the bread also which I will give FOR THE LIFE OF THE WORLD IS MY FLESH."

Jumping to vs55-56, "He that eateth--and drinketh indicates a present tense habitual communion with Jesus. "Dwelleth in me" should have convinced the Jews and the disciples that Jesus was not speaking in literal terms.

Two verses come to mind. John 14:23, "If anyone loves Me, he will keep My word, and My Father will love him, and We will come to him, and make Our ABODE with him." Also, Revelation 3:20, "Behold I stand at the door and knock, if anyone hears My voice and opens the door, I will come in to him, and will dine with him, and he with Me."

John 6:58 is a synopsis of the argument the Jews had with Jesus. They had asked for a sign similar to the manna in the desert wanderings (verses 31,32, 49). The contrast is between their fathers who ate the manna and died, and those who eat of Jesus and live forever. The two words "not as" in this verse shows the difference between the bread Jesus gives and the manna in the wilderness.

And yes, it is hard to accept, (vs60). At vs61, Jesus knew what they were thinking and at vs62 He says, "What then if you should behold the Son of Man ascending where He was before?"

In other words, you ain't seen nothing yet? If your stumbling at My descent from heaven, wait until you see Me ASCENT back to heaven after My crucifixion and death.

From my perspective as a Protestant Christian, the reason why some people took offense at Jesus's words about eating his flesh and consequently no longer followed Jesus is because they were being too literal in their interpretation of Jesus's words. They took him literally; Jesus was speaking spiritually:

The Spirit gives life; the flesh counts for nothing. The words I have spoken to you—they are full of the Spirit[e] and life. 64 Yet there are some of you who do not believe (John 6:63-64 NIV).

May I suggest that Jesus offended some of his listeners deliberately, for the same reason he told parables to most of his listeners. His modus operandi was to winnow the number of followers/disciples whose hearts were not really committed to discipleship. They were not willing to be Jesus's students/pupils/learners and faithful followers. For them, the cost of discipleship was too much. To paraphrase Jesus's words, they had ears but they didn't hear; they saw, but they didn't see. Put differently, they found they couldn't connect--or were unwilling to connect--faith with works.

In conclusion, Jesus's use of the metaphors of flesh and blood was not literal; rather, his use of them was spiritual. Jesus was not encouraging his followers to become cannibals! This is where Protestant theology conflicts with Roman Catholic theology. In the celebration of the eucharist, Protestants believe the bread and wine are symbols only; they stand for--or take the place of--the body and blood of their Lord and Savior. They partake of the elements "in remembrance of" Jesus (Luke 22:19; 1 Corinthians 11:24-25).

As for what Roman Catholics believe about the elements of the communion, I'm not really sure. The "presence" you talk about may be linked to their belief in transubstantiation, though I am not really sure. One thing is for sure, when Roman Catholics partake of the elements, they do not have literal flesh and blood dangling from and dripping down their chins. They know that, of course, which proves, to me at least, they know Jesus's words about eating his flesh and drinking his blood were spiritual, not literal.

There are two threads in this passage. One is a "You can't come unless the Father draws you" thread and the other is a "bread of life" thread and the two threads are co-mingled. The entire passage is Jesus trying to speak truth to the dull of hearing. As dullness of hearing turns into hardness of heart, so Jesus' teaching becomes more parabolic in keeping with His stated reason for using parables.

As regards the "bread of life" thread, the scene starts with the feeding of the 5 thousand, followed by the same crowd that He fed meeting Him across the lake, followed by this chastis*m*nt of the crowd:

Jesus answered them and said, Verily, verily, I say unto you, Ye seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled. Labour not for the meat which perisheth, but for that meat which endureth unto everlasting life, which the Son of man shall give unto you: for him hath God the Father sealed. - John 6:26-27

Jesus first point is directing them away from the physical to the spiritual. "You came because I filled your bellies with temporal provision but I want to fill your spirits with life."

Since Jesus has used the word labour the crowd asks "What work must we do?" and Jesus responds "Believe in me (the one God has sent)."

Why should we believe?, the crowd asks, What sign will you show us so that we can believe? Our fathers ate bread in the desert, what are you gonna do?

Jesus - Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven. For the bread of God is he which cometh down from heaven, and giveth life unto the world.

Crowd - Give us that bread.

Jesus - I am the bread of life: he that cometh to me shall never hunger; and he that believeth on me shall never thirst.

Notice that, at this point, hunger is quenched by coming and thirst is quenched by believing. The crowd is still looking for physical food and Jesus is trying to point to spiritual provision. Next it is the Jews in the crowd who begin to murmur:

The Jews then murmured at him, because he said, I am the bread which came down from heaven. And they said, Is not this Jesus, the son of Joseph, whose father and mother we know? how is it then that he saith, I came down from heaven? - John 6:41-42

Notice that the Jews problem now is not the bread thing but the origin of Jesus. They are rejecting who He is and where He is from which was the whole point of the feeding of the crowd in the first place.

Jesus - Don't murmur. If you won't respond to the Father's drawing you will not understand. I am the bread of life and if you believe in me you have everlasting life. (vs. 43-48)

Now Jesus begins speaking parabolically to the Jews who have begun murmuring against what has been intended to draw them to the truth.

Jesus - Your fathers ate bread and they are dead. I am the bread you can eat and live forever. My body is the true manna from heaven. (vs. 49-51)

Jews - You want us to eat your flesh?!? (vs. 52)

Jesus - If you don't eat my flesh and drink my blood you have no life in you. (vs. 53-58)

It is important to recognize that the crowd (Jews included) were first instructed to come and believe for life and that the eating and drinking metaphors were introduced to those who were already in the process of rejecting Jesus' self revelation.

Later his disciples (some of whom did not believe (v. 64)) express concern over the hard teaching and Jesus says "Really, how offended are you going to be when I go back where I came from. There is no profit in the flesh. If you're not being drawn by my Father you can't come to me" (vs.62-65). At this point many disciples leave.

The entire passage is one of tension between the fleshly focus upon the physical and Jesus trying to use the physical to spotlight the spiritual. That was the purpose of the feeding of the multitude and his conversation with them. It was to those in that crowd who were rejecting the authentication of who He is that he used the eating and drinking language.

And so it is today those same, amongst the crowd who cannot quite reach that place where Jesus is actually, completely sufficient in all aspects and in every way, who must position physical religious requirements within hierarchical limitations accompanied by strange teachings which detract from the simplicity that is in Christ.

Come and believe is the means of salvation for those with ears to hear.

Eat my flesh and drink my blood is the parable to those without.