At the opposite end of the scale, the 'sinners of the Gentiles' have a difficult time 'tightening up' on trying to follow God's Law which remains in effect. All in all, God's Law is good. But only through faith in Christ Jesus are we 'justified', or 'saved’ and that by His unmerited favour of Grace.
     In Peter's case, he was being a little 'two-faced', supping with Gentiles only until his Jewish brethren arrived. No doubt this event by Peter was for our learning; for both Jew and Gentile, much as Peter denied Christ three times before the cock crowed.

 

 Matt.26:33-34; 69-74: Read All. 

 

What's more, the vision Peter was given by God concerning Salvation to the Gentile, and what God cleansed, we are not to call common, he should have set an example for his Jewish brethren when among the Gentiles at Antioch, instead of falling back into 'old habits'. 
     That's the Message for us. After having accepted Christ as Saviour, we often tend to fall back into old routines if we aren't careful, because flesh man naturally loves habit.

 The flesh prefers permanence, not change. And if we don't stay awake in God's Word, we can easily fall prey to false doctrines and traditions which Satan purposefully designs to test our flesh habits, and this concerns any habits of idol worship also.

 

Gal 2:17:  But if, while we seek to be justified by Christ, we ourselves also are found sinners, is therefore Christ the minister of sin? God forbid."
18    For if I build again the things which I destroyed, I make myself a transgressor.

     So because we are now saved by the Grace of Christ, and not by The Law, does that mean we can just go out and commit sin, knowing all we have to do is believe on Jesus and repent?   God forbid, no! As said before...

God's Law is embedded in the world of flesh. If we break it, we will pay, but by sincere repentance and attempting to change, we are justified by our faith in Jesus Christ. Those who go and 'knowingly' sin because they think all they've got to do is repent, will receive a double-portion of punishment from God .

 

Heb.10:26:  For if we sin wilfully after that we have received the knowledge of the truth, there remaineth no more sacrifice for sins.

 

 Rev.18:6. Reward her even as she rewarded you, and double unto her double according to her works: in the cup which she hath filled fill to her double.

     If it was pork those Gentiles were eating, and Peter partook, since pork is against God's Health Law, does that mean Peter would lose salvation through Christ? No, but Peter's flesh body wouldn't be very healthy if he kept eating it. The same goes for the Gentile, as all of us are under God's Law, not only Jews. God gave The Law to us to keep our flesh tuned-up to feel good, as our Father created these flesh bodies, and He knows what's best for them. Some people have a bad reaction to a little alcohol. It's a medical fact that a little wine is good for the flesh system (a little means like one glass with a main meal). Will they go to hell if they drink 'a little' for the stomach? No. But those who abuse alcohol, etc., they will pay for their indulgence, and usually in more ways than one. God's Law governs the reward in staying within His Law, or the punishment in transgressing It, in either case.
     Paul is saying in verse 18 that, if we go back to relying only upon God's Law to be our Salvation, and not Jesus Christ as our Saviour, then we become transgressors of Grace and The Gospel of Jesus Christ. That's what Peter's actions represented, by eating with the Gentiles at first and then separating himself according to that old tradition when his Jewish brethren arrived. Peter should have been aware of his actions, especially since God gave him the Message that the Gentile was to be included in Salvation. This is why Paul came down on him so hard, because Peter was entrusted with a lot from God.