How can I convince a Seventh-day Adventist Christian that Sunday is the holy day of rest and not Saturday?
You do not have to convince him. As long as someone does not consider the observing of the Sabbath or of the day of rest as a condition for salvation of the soul we are advised to not judge about questionable opinions. The day of rest belongs to this category of questionable opinions. In the Epistle to the Romans, the Apostle Paul wrote:
Now accept the one who is weak in faith, but not for the purpose of passing judgment on his opinions. One person has faith that he may eat all things, but he who is weak eats vegetables only. The one who eats is not to regard with contempt the one who does not eat, and the one who does not eat is not to judge the one who eats, for God has accepted him. Who are you to judge the servant of another? To his own master he stands or falls; and he will stand, for the Lord is able to make him stand. One person regards one day above another, another regards every day alike Each person must be fully convinced in his own mind. He who observes the day, observes it for the Lord, and he who eats, does so for the Lord, for he gives thanks to God; and he who eats not, for the Lord he does not eat, and gives thanks to God. (Romans 14:1-6)(NASB)
Translated by Felicia Rotaru