Disclaimer: This is my own idea -- I don't know if any authority proposed or espouses this. I feel strongly that this is true, but it's only my feeling.
I speak often to my best friend, Michelle Nevada, since we don't live in the same zip, area or even Morse code. We often have very interesting conversations and oftentimes, talking to her helps me put my opinions and ideas into words. Today was no exception to this.
We were talking about religion and Judaism and what it is that makes someone a "good Jew". And I said that keeping Shabbat and keeping kosher are not what make you a good Jew. Those are G-d's gifts to us. What makes you a "good Jew" is how you treat other people. In other words, if you treat people with respect and do what you can to help others you are a good Jew even if you never kept Shabbat in your life.
There are two major groupings of Mitzvot (commandments) -- those between people and other people (called in Hebrew Bein Adam L'Haveiro -- between man and his friend) and those between people and G-d (called in Hebrew Bein Adam L'Makom -- between man the Omnipresent, or G-d).
The laws between people and other people are the laws about how to treat other people, like helping others, giving charity (called Tzedaka from the Hebrew word for justice), treating others with respect, things like that. The laws between people and G-d are laws like keeping kosher, keeping Shabbat, putting up a Sukka and things like that.
The laws between people and other people are the laws that make getting along in this world, the world G-d created, easier. If we treat others with respect, they will respect us. If we help others, they will help us. If we are honest with people, they will trust us and be honest with us. Keeping these laws are what makes you a "Good Jew", a good person, a righteous person.
Shabbat and other similar laws, on the other hand, are G-d's gift to us. How many people do we know who are "on" 24/7, always available for work, always checking the stock market. I know myself that without Shabbat I would probably be on my computer all the time. But by taking 25 hours off for Shabbat, I have an opportunity to recharge my batteries. Shabbat is the origin of the "weekend".
So, if you are Jewish and think that you're not a good Jew because you go shopping on Saturday or eat ham sandwiches, I'll tell you you're mistaken. You're a good Jews if you are helping others. You are missing out on G-d's gift to you if you go shopping on Saturday or eat ham sandwiches. If you are a Gentile, you're a good person if you help others and keep the laws between people and other people (Gentiles are not permitted to keep Shabbat the way Jews do because Shabbat is a covenant between G-d and the Jewish people). If you will check out the 7 Noachide Laws (the laws that Gentiles are responsible for), you will notice they are all laws of how you interact with other people and other sentient creatures. These are the laws that make one a Good person, Jew or Gentile.
If you are a Jew and you want to begin to keep the laws in both categories, you start with a clean slate; you can decide at any moment you would like to accept the gifts that G-d has given us. You don't have to do anything other than what Nike says, "Just do it!", to make G-d proud.