Ellen White calls it the final test, "...Satan’s war against God and His authority has focused upon the character of God and His law from the very beginning, and it will escalate just before Jesus comes the second time.
Jesus must have known that Satan would tempt us to doubt our need to obey God’s law, for He emphasized that our obedience is a demonstration of our love to Him: “If you love Me, keep My commandments. . . . If you keep My commandments, you will abide in My love, just as I have kept My Father’s commandments and abide in His love.” John 14:15; 15:10. John the beloved wrote, “For this is the love of God, that we keep His commandments. And His commandments are not burdensome.” 1 John 5:3.
The fourth of the ten commandments reminds us to keep the seventh-day Sabbath:
“Remember the Sabbath day, to keep it holy. Six days you shall labor and do all your work, but the seventh day is the Sabbath of the Lord your God. In it you shall do no work: you, nor your son, nor your daughter, nor your male servant, nor your female servant, nor your cattle, nor your stranger who is within your gates. For in six days the Lord made the heavens and the earth, the sea, and all that is in them, and rested the seventh day. Therefore the Lord blessed the Sabbath day and hallowed it.” Exodus 20:8-11...."
Is it what shows who they follow and separate the saved from the lost?
Satan has founded many things to lead us away from the True God. Had the Sabbath been kept all the time EGW says there would not be any atheism because men were lead away from biblical truths they were looking for other reasons for life and creation. The answer to life has not been the atheist's strong suit as they have not been able to create life in their experiments, or have found a convincing answer apart from God as the creator. The question about the True God will be tested out as we near the end as we read in Revelation.