I actually do think and know that the grace of God is greater than any sin as Christ has defeated it for us. I believe that it is only the teaching of the grace of God that is found in the gospel that has the power for salvation.
I believe as we grow in the Lord - the things that beset us and are deceiving us will fall off of us so I don't agree with the notion that sin is stronger than Christ. It's foolishness to my way of thinking. I believe the complete opposite.
I think the reason many here refute the "keeping of the law" for righteousness and salvation is because it actually negates the work of grace in our lives. Grace will teach us how to live godly in this world. - not the teaching or keeping of the law.
The law is the knowledge of sin - we all know within us even people that have never heard of Moses that it is wrong to steal and murder..etc.
Where the rubber meets the road in this law vs grace debates is this: If you could answer this then maybe a lot of this stuff is moot and we are dancing around the same song but with different words.
"Is a person that does not observe the Sabbath day as in the law of Moses from Friday to Saturday or observe the feast days as outlined in the law of Moses. - If the Christian doesn't do these things - are they dis-obeying God and sinning?"
Do we have to observe feast days as outlined in the law of Moses? As I am aware no as these are shadows of the work of Christ as Messiah. While they have valuable lessons within them to help us understand Jesus and know Him and his redemption better.
I am going to assume given context that the Sabbath you here refer to is not the feast Sabbaths but the Sabbath of the Ten Commandments. All feast Sabbaths were finished at the cross. This is due to the fact that These again are shadows of the work of Christ as Messiah.
The 7th day Sabbath though is in the 10 commandments. What does that tell us about it?
Rom 3:19 Now we know that what things soever the law saith, it saith to them who are under the law: that every mouth may be stopped, and all the world may become guilty before God.
Rom 3:20 Therefore by the deeds of the law there shall no flesh be justified in his sight: for by the law is the knowledge of sin.
The fact that we know that the 10 commandments show what sin is and this is clear contextually that the 10 commandments are being spoken of here. Then the Sabbath commandment also like the other nine shows what is sin by that which is good. Notice also that Paul clearly teaches that the "world" may become guilty. This is not just for Jews. A law that does not apply to Gentiles can not make them guilty or show them sin. Thus it is clear contextually that all whether Jew or Gentile are under the law before they find Christ. Thus the Sabbath commandment applies just as truly as the other nine to all the inhabitants of the world.