Could you tell me why the Sunday-Law of Constantine is ignored as if Sunday is mentioned in the Greek Scriptures. Is this not a turning point of the history of the apostasy of the church as prophesied in 2nd Thessalonians 2?

I assume that you are referring to the law Constantine passed in 321 AD that closed all courts of law on Sunday and placed restrictions on the use of slave labor except for certain farming activities. He also changed the market day for farmers to Sunday, encouraging Romans not to work everyday of the week. These two changes pushed the culture into observing some kind of "sabbath" day of rest. This was not an enforced law, but a way to encourage Sunday as a day for worship. When he passed these laws he recognized them as "the day of the sun," and was not forcing anyone to attend Christian church.

Christians started worshipping on Sunday to observe "The Lord's Day" early in the first century. Jewish Christians celebrated the Sabbath day of rest AND met for Christian worship on Sunday mornings. Gentile believers worshipped only on Sunday unless they happened to be in a mainly Jewish church - but this was not easy since Gentiles were not openly welcomed in the Temple or tabernacles. They were more welcome IF they were attempting to convert to Judaism, but as Christians this would NOT be a viable option (Paul's letter to the Galatians makes this quite clear).

Early in the second century (after the destruction of the Temple in 70 AD and Jewish worship taking a heavy hit) the mainly Gentile church began to show anti-Jewish sentiments. Many believed that the destruction of the Temple was God's judgement against Israel and you can read an anger towards Jews (read comments on Barnabas; Marcion also reflects this attitude).

Worship on Sunday, the Eighth Day

I realize that many will contest what I have just written, but there are numerous examples from church fathers in the first and second centuries which make it clear that Christians met on the eighth day to commemorate the resurrection day of Jesus. There was a clear anti-Jewish strain in the early Gentile church - I am not saying this is good, but it is historically true.

You can see the promtion of Sunday AND the anti-Jewish sentiment in this text.
The Didache also speaks of worship on the Lord's day, a reference to Sunday (Didache 14:1). Ignatius of Antioch (cir 112-120 AD) also speaks of worship on Sunday AND reveals this anti-Jewish sentiment:

Greetings ! Thank you for your kind reply and I need to listen to you fully when you can fully explain Scripturally, in due course. Thanking you, Yours in His service,
M.P.S.Raju

As you well know, there is no clear evidence from the scriptures for worship on Sunday just as there is no clear evidence that Christians met in church buildings, no clear evidence in the NT for tithing, no clear evidence for musical instruments being used in worship, Yet there IS evidence for women wearing a head covering, being silent in church, baptism for the dead, consumption of alcoholic beverages, and possibly even polygamy...

The point is that the New Testament does not contain ALL information about everything. No matter what tradition you are in I can find something that you believe that is NOT clearly established in the New Testament. And this is the danger of trying to hold to a literalist position with the biblical text. Many of the church fathers struggled with this - it is nothing new. The biblical text is not meant to be held together without tension. This explains why the Church has always had men who have disagreed in their interpretations on the text. Those who demand a literal reading of everything must strain in the their interpretations (and logic) in order to try to present a position without conflicting data.

I realize that you may strongly disagree with me here, but that is my opinion and my position. Happy to hear yours.