Every false worship claim to be of God but a careful study reveals the deception attached to it. People say the day you worship God doesn't matter but God's word also says God identifies His people by worship. The Great Controversy going on between Christ and Satan is about worship. This clearly reveals there's true and false worship. God's commands are plain and He says "Remember the sabbath day, to keep it holy." Exodus 20:8
The seventh day is the sabbath of the LORD God almighty and in it He identifies His people. The Sabbath is a love relationship between God and His people. Saturday is the sabbath day and Satan showing his hatred against it has cast contempt upon it as a Jewish tradition but it's false. The Sabbath is for all humanity and Jesus reenforced the sabbath observance. He said "The sabbath was made for man(This means all humanity and not just jews!)… Therefore the Son of man is Lord also of the sabbath. Mark 2:27-28"
Jesus and His disciples kept the sabbath, Paul and the early Christians also kept the sabbath. The sabbath was kept throughout time by God's people until something happened: "Daniel 7:25 And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time." This prophecy was fulfilled after the ascension of Christ and the death of the apostles. An evil power came into power which taught to change God's law. The Roman power made the sabbath change and they freely and boastfully admit it.
Emperor Constantine on March 7, 321 AD passed a decree enforcing a working day instead of God's Holy day to be a day of worship. Sunday is the first day of the week and a common working day. It bears no sanctity and not in the Bible as a sabbath day. The Roman Catholic Church, the Antichrist power made this change. The little horn in Daniel 7 which is the papal power boastfully presumed the titles which only belongs to God!
Sun worship is paganism and started long ago by Nimrod. This is what the Catholic Church confesses about Sunday worship being paganism: “…pastoral intuition suggested to the Church the christianization of the notion of Sunday as “the day of the sun”, which was the Roman name for the day and which is retained in some modern languages.(29) This was in order to draw the faithful away from the seduction of cults which worshipped the sun, and to direct the celebration of the day to Christ, humanity’s true “sun”.” John Paul II, Dies Domini, 27. The day of Christ-Light, 1998 (Prominent protestant leaders agree with this statement – See here for a statement by Dr. E. T. Hiscox, author of the ‘Baptist Manual’)
“The Sun was a foremost god with heathen-dom…The sun has worshippers at this hour in Persia and other lands…. There is, in truth, something royal, kingly about the sun, making it a fit emblem of Jesus, the Sun of Justice. Hence the church in these countries would seem to have said, to ‘Keep that old pagan name [Sunday]. It shall remain consecrated, sanctified.’ And thus the pagan Sunday, dedicated to Balder, became the Christian Sunday, sacred to Jesus.” William Gildea, Doctor of Divinity, The Catholic World, March, 1894, p. 809
“The retention of the old pagan name of Dies Solis, for Sunday is, in a great measure, owing to the union of pagan and Christian sentiment with which the first day of the week was recommended by Constantine to his subjects – pagan and Christian alike – as the ‘venerable’ day of the sun.”” Arthur P. Stanley, History of the Eastern Church, p. 184
The Roman Catholic Church is the Antichrist System and God is calling His people out of this erroneous organisation before it's too late. God has many sincere people in Babylon(Roman Catholicism and apostate protestantism) and they must come out of Babylon. Sunday worship is the mark of the beast.
Of Babylon, at the time brought to view in this prophecy, it is declared: “Her sins have reached unto heaven, and God hath remembered her iniquities.” Revelation 18:5. She has filled up the measure of her guilt, and destruction is about to fall upon her. But God still has a people in Babylon; and before the visitation of His judgments these faithful ones must be called out, that they partake not of her sins and “receive not of her plagues.” Hence the movement symbolized by the angel coming down from heaven, lightening the earth with his glory and crying mightily with a strong voice, announcing the sins of Babylon. In connection with his message the call is heard: “Come out of her, My people.” These announcements, uniting with the third angel’s message, constitute the final warning to be given to the inhabitants of the earth.
Fearful is the issue to which the world is to be brought. The powers of earth, uniting to war against the commandments of God, will decree that “all, both small and great, rich and poor, free and bond” (Revelation 13:16), shall conform to the customs of the church by the observance of the false sabbath. All who refuse compliance will be visited with civil penalties, and it will finally be declared that they are deserving of death. On the other hand, the law of God enjoining the Creator’s rest day demands obedience and threatens wrath against all who transgress its precepts.
With the issue thus clearly brought before him, whoever shall trample upon God’s law to obey a human enactment receives the mark of the beast; he accepts the sign of allegiance to the power which he chooses to obey instead of God. The warning from heaven is: “If any man worship the beast and his image, and receive his mark in his forehead, or in his hand, the same shall drink of the wine of the wrath of God, which is poured out without mixture into the cup of His indignation.” Revelation 14:9, 10.
But not one is made to suffer the wrath of God until the truth has been brought home to his mind and conscience, and has been rejected. There are many who have never had an opportunity to hear the special truths for this time. The obligation of the fourth commandment has never been set before them in its true light. He who reads every heart and tries every motive will leave none who desire a knowledge of the truth, to be deceived as to the issues of the controversy. The decree is not to be urged upon the people blindly. Everyone is to have sufficient light to make his decision intelligently.
The Sabbath will be the great test of loyalty, for it is the point of truth especially controverted. When the final test shall be brought to bear upon men, then the line of distinction will be drawn between those who serve God and those who serve Him not. While the observance of the false sabbath in compliance with the law of the state, contrary to the fourth commandment, will be an avowal of allegiance to a power that is in opposition to God, the keeping of the true Sabbath, in obedience to God’s law, is an evidence of loyalty to the Creator. While one class, by accepting the sign of submission to earthly powers, receive the mark of the beast, the other choosing the token of allegiance to divine authority, receive the seal of God.
Heretofore those who presented the truths of the third angel’s message have often been regarded as mere alarmists. Their predictions that religious intolerance would gain control in the United States, that church and state would unite to persecute those who keep the commandments of God, have been pronounced groundless and absurd. It has been confidently declared that this land could never become other than what it has been—the defender of religious freedom. But as the question of enforcing Sunday observance is widely agitated, the event so long doubted and disbelieved is seen to be approaching, and the third message will produce an effect which it could not have had before.
Through the two great errors, the immortality of the soul and Sundaysacredness, Satan will bring the people under his deceptions. While the former lays the foundation of Spiritualism, the latter creates a bond of sympathy with Rome. The Protestants of the United States will be foremost in stretching their hands across the gulf to grasp the hand of Spiritualism; they will reach over the abyss to clasp hands with the Roman power; and under the influence of this threefold union, this country will follow in the steps of Rome in trampling on the rights of conscience.
Results of not heeding God's warning: Many of the wicked were greatly enraged as they suffered the effects of the plagues. It was a scene of fearful agony. Parents were bitterly reproaching their children, and children their parents, brothers their sisters, and sisters their brothers. Loud, wailing cries were heard in every direction, “It was you who kept me from receiving the truth which would have saved me from this awful hour.” The people turned upon their ministers with bitter hate and reproached them, saying, “You have not warned us. You told us that all the world was to be converted, and cried, Peace, peace, to quiet every fear that was aroused. You have not told us of this hour; and those who warned us of it you declared to be fanatics and evil men, who would ruin us.” But I saw that the ministers did not escape the wrath of God.Their suffering was tenfold greater than that of their people. Early Writings 282.1
The Great Controversy, The Great Controversy, 588. Christian Service 160.1