If you say I am wrong about your statements: Will you agree that we do not need to observe the OT Sabbath, and that not observing the OT Sabbath (as you say the 4th command) is not practicing sin or living an unrepentant life?
Would you agree with that?
You need to understand what the OLD Covenant is and what the NEW Covenant is. You need to understand the PURPOSE of God's 10 Commandments in the OLD and NEW Covenants. There pupose is to give us a KNOLWEDGE of what SIN and RIGHTEOUSNESS IS (Good and Evil). God's LAW leads us to Jesus who is the Saviour from SIN in the NT.
God's LAW makes nothing perfect it only shows us what SIN is if we break it and what RIGHTEOSNESS is if obeyed. ALL the 10 Commandments are repeated in the NEW Testament. THey are a part of the NEW Covenant (Hebrews 8:10-12) but through FAITH we are BORN AGAIN to LOVE and LOVE is the fulfilling (DOING) of God's LAW for those who accept the GIFT of God's GRACE through FAITH to LOVE another.
If we break ANY of the 10 Commandments we stand GUILTY before GOD of SIN and do not LOVE him who calls us in LOVE to LOVE another. If those who CONTINUE in KNOWN SIN do NOT REPENT and CONTINUE in SIN they will NOT enter into God's KINGDOM
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Just like any of the 10 Commandments; God's 4th commandment is one of the ten (Exodus 20:8-11) If we knowingly break it when God asks us not to we stand guilty before God of committing sin (James 2:8-12). If we do not seek him in repentance and forgiveness we are in danger of the Judgement (Hebrews 10:26-27)
Sunday worship is a tradition and teaching of man that has lead many to break the commandments of God. Jesus says that if we follow the traditions of man that break the commandments of God we are not following God (Matthew 15:3-9)
There is not one scripture in all of God's Word that says that God's 4th Commandment is now ABOLISHED and we are now commanded to KEEP Sunday as a Holy day.
Who should we follow the teachings and traditions of men or the Word of God? Who should we believe the Words of men or the Word of God?
In times of ignorance God winks at but now ,<when a KNOWLEDGE of the truth has come> calls all men everywhere to REPENT (FOLLOW) (Acts 17:30-31).