Biblical explanation
The Sabbath begins at creation: "And on the seventh day God ended his work which he had made... And God blessed the seventh day, and sanctified it." Genesis 2:2-3 The fourth commandment says: "Remember the sabbath day, to keep it holy." Exodus 20:8 It specifically identifies the seventh day as the Sabbath. Jesus did not present the Sabbath as something that existed only temporarily for Jews. He said: "The sabbath was made for man." Mark 2:27 He also called Himself: "Lord also of the sabbath." Mark 2:28 The New Testament continues to describe Jesus and His followers participating in Sabbath observance. Scripture clearly commands the seventh-day Sabbath and records no verse saying that Sunday replaced it.
Key takeaway
Salvation is entirely by grace through faith in Christ. His atoning sacrifice, heavenly ministry, and intercession are the foundation of assurance; obedience, Sabbath faithfulness, and loving service are the fruit of a redeemed life, never a way to purchase acceptance with God.