Sin is an affront to God's character, which has not changed between testaments, so everything that is a sin in the NT is a sin in the OT and vice versa. Violating God's dietary laws is also an affront to His character and is just as much a sin today as it was a sin back then. God has given instructions in His law for how to live rightly, and if we have faith in Him, then we will obey them. Jesus gave no indication that was he was in disagreement with the Father about which laws we should follow, but rather he said he came only to do the Father's will (
John 6:38) and that his teachings were not his own, but that of the Father (
John 7:16), so there is no difference between the law of Christ, the law of the Father, and the law of the Spirit. Jesus taught nothing that was brand new or took away from the OT, but only taught how to correctly understand and follow the God's law. Jesus said nothing about editing God's law or teaching a brand new law, but rather he said that not the least part of the law would disappear and warned those (like you) who would relax the least command or teach others to do the same (
Matthew 5:17-19).
You misunderstand what Christ means when he says that His word belongs to God. Christ is not telling us that hes commanding us to live under OC law. Israel broke Gods covenant and he made anew one, which includes Gentiles as well. Look at Mathew 5-7, which you cited. You wont see Sabbath mentioned anywhere, nor any dietary laws. Christ actually says that its not what goes in the mouth which defiles but what comes out that defiles, because that comes from mans heart.
In my recent studies of scripture, i have uncovered a great deal of scripture that teaches about how we use our words. This seems to be a huge part of the meat in the word. I could probably find many more as well. I see this in Isaiah when God says that we should refrain from using our words, from speaking wickedness, i see it in all of Paul writings where he teaches us to put away our wicked speech and speak in love. Its not what goes in our mouth which defiles, but what comes out. This is why Paul says that all things are pure, referring to food.
Mathew 15:11 Not what goes into the mouth defiles a man; but what comes out of the mouth, this defiles a man
Isaiah 58:“If you take away the yoke from your midst,
The pointing of the finger, and
speakingwickedness,
Isaiah 58: Nor finding your own pleasure,
Nor
speaking your own words,
2 Peter 1:22 “Who committed no sin,
Nor was deceit found in His mouth”; 23 who, when He was reviled,
did not revile in return; when He suffered, He did not threaten, but committed Himself to Him who judges righteously; 24 who Himself bore our sins in His own body on the tree, that we, having died to sins, might live for righteousness—by whose stripes you were healed. 25 For you were like sheep going astray, but have now returned to the Shepherd and Overseer
of your souls.
Galtians 5:15
But if you
bite and devour one another, beware lest you be consumed by one another!
16 I say then: Walk in the Spirit, and you shall not fulfill the lust of the flesh.17 For the flesh lusts against the Spirit, and the Spirit against the flesh; and these are contrary to one another, so that you do not do the things that you wish. 18 But if you are led by the Spirit, you are not under the law.
19 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery,
hatred,
contentions, jealousies, outbursts of wrath, selfish ambitions,
dissensions, heresies, 21 envy, murders, drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told
you in time past, that those who practice such things will not inherit the kingdom of God.
22
But the fruit of the Spirit is
love, joy, peace, longsuffering, kindness, goodness, faithfulness, 23 gentleness, self-control. Against such there is no law. 24 And those
who are Christ’s have crucified the flesh with its passions and desires. 25 If we live in the Spirit, let us also walk in the Spirit. 26 Let us not become conceited, provoking one another, envying one another.
1 Co 13:8
Love never fails. But whether
there are prophecies, they will fail; whether
there are tongues, they will cease; whether
there is knowledge, it will vanish away. 9 For we know in part and we prophesy in part. 10 But when that which is perfect has come, then that which is in part will be done away.
11 When I was a child,
I spoke as a child, I understood as a child, I thought as a child; but when I became a man, I put away childish things. 12 For now we see in a mirror, dimly, but then face to face. Now I know in part, but then I shall know just as I also am known.
13 And now abide faith, hope, love, these three; but the greatest of these
is love.