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I was brought up in a denomination wherein the Lord's Supper was called
Communion, and we consumed Jesus' flesh and blood by means of a process
called transubstantiation; in accord with John 6:53-58.
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You should prove all things with Jesus Christ because what Jesus was talking about in John 6:28-71 was about how we are saved by coming to and believing in Jesus Christ; that is how they were to receive the bread of life.
John 6:28 Then said they unto him, What shall we do, that we might work the works of God? 29 Jesus answered and said unto them, This is the work of God, that ye believe on him whom he hath sent.
30 They said therefore unto him, What sign shewest thou then, that we may see, and believe thee? what dost thou work? 31 Our fathers did eat manna in the desert; as it is written, He gave them bread from heaven to eat.
32 Then Jesus said unto them, Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven. 33 For the bread of God is he which cometh down from heaven, and giveth life unto the world.
34 Then said they unto him, Lord, evermore give us this bread.
35 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. 36 But I said unto you, That ye also have seen me, and believe not.
They had asked Jesus fir that bread of life and He told them how to receive that bread of life by coming to & believing in Jesus Christ but they did not believe Him.
The Jews were stuck in that mindset of eating manna from heaven like their forefathers but no. So the verses you cited for transubstantiation is misapplied because Jesus was just referring to His crucifixion; not communion. Look a John 6:35 as proof that He was not talking about communion for that promise is to never thirst nor hunger again and so if the RCC applied your reference to mean communion, then they would only have to take communion one time and that is it, but no.
Jesus explained it plainly to His disciples that the flesh profiteth nothing in verse 63 for He was talking about how we are saved and even inferred His ascension which was to take place after His crucifixion in verse 62.
John 6:61 When Jesus knew in himself that his disciples murmured at it, he said unto them, Doth this offend you? 62 What and if ye shall see the Son of man ascend up where he was before? 63 It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life. 64 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
It is by coming to and believing in Jesus Christ is how we are saved; not by coming to the Mass for receiving the one time sacrifice for sins again to receive atonement for sins since last Mass. That would be insulting the blood of the New Covenant for putting His blood on par with the blood of goats and bulls that it bears repeating again. It does not, because the Holy Spirit in believers is proof of that.
Hebrews 10:3 But in those sacrifices there is a remembrance again made of sins every year. 4 For it is not possible that the blood of bulls and of goats should take away sins.
9 Then said he, Lo, I come to do thy will, O God. He taketh away the first, that he may establish the second. 10 By the which will we are sanctified through the offering of the body of Jesus Christ once for all. 11 And every priest standeth daily ministering and offering oftentimes the same sacrifices, which can never take away sins: 12 But this man, after he had offered one sacrifice for sins for ever, sat down on the right hand of God; 13 From henceforth expecting till his enemies be made his footstool. 14 For by one offering he hath perfected for ever them that are sanctified. 15 Whereof the Holy Ghost also is a witness to us: ............
So there is a danger to Catholics continuing in the Mass for why they should stop doing that and only do it in remembrance of Him which is anathema to the RCC for why saved believers should leave the RCC.
That sin willfully part is about having knowledge that there is no more sacrifice for sins to be had or received and yet they are sinning willfully in the Mass.
Hebrews 10:26 For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins, 27 But a certain fearful looking for of judgment and fiery indignation, which shall devour the adversaries. 28 He that despised Moses' law died without mercy under two or three witnesses: 29 Of how much sorer punishment, suppose ye, shall he be thought worthy, who hath trodden under foot the Son of God, and hath counted the blood of the covenant, wherewith he was sanctified, an unholy thing, and hath done despite unto the Spirit of grace? 30 For we know him that hath said, Vengeance belongeth unto me, I will recompense, saith the Lord. And again, The Lord shall judge his people. 31 It is a fearful thing to fall into the hands of the living God.
Catholics are still saved and still His people for believing in the Lord Jesus Christ and that God has raised Him from the dead, but the Mass is a work of iniquity.
It does not matter if they believe Jesus is "willing" to make that one time sacrifice for sins "present" for them to receive again because that is the same thing as saying His blood was not good enough the first time when shed on the cross.
The fact that all Catholics that believe in the Lord Jesus Christ & that God has raised Him from the dead are saved thus born again of the Spirit, thus having the Holy Spirit in them is evidence that there is no more sacrifice for sins to be received "any more".
All believers has to do for the forgiveness of sins is go to Jesus Christ and ask Him for it & His help not to commit that sin again because they are His and thus saved.
1 John 1:9 If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness.
That is the truth as found in the scriptures and may those that fear Him, heed His words to repent and rejoice and hold communion in remembrance of what He has done; what He has accomplished as "it is finished" in remembering * proclaiming the Lord's death till He comes.