It never says the son of the foreigner keeps the Shabbat.
Until you read the text -and note that "everyone" includes the "son of the foreigner" in vs 6.
Isaiah 56
Blessed
is the man
who does this,
And the son of man
who lays hold on it;
Who keeps from defiling the Sabbath,
And keeps his hand from doing any evil.”
3 Do not let
the son of the foreigner
Who has joined himself to the Lord
Speak, saying,
“The Lord has utterly separated me from His people”;
Nor let the eunuch say,
“Here I am, a dry tree.”
4 For thus says the Lord:
“To the eunuchs
who keep My Sabbaths,
And choose what pleases Me,
And hold fast My covenant,
5 Even to them I will give in My house
And within My walls a place and a name
Better than that of sons and daughters;
I will give them an everlasting name
That shall not be cut off.
6 “Also the
sons of the foreigner
Who join themselves to the Lord, to serve Him,
And to love the name of the Lord, to be His servants—
Everyone who keeps from defiling the Sabbath,
And holds fast My covenant—
7 Even them I will bring to My holy mountain,
And make them joyful in My house of prayer.
Their burnt offerings and their sacrifices
Will be accepted on My altar;
For
My house shall be called a house of prayer for all nations.”
"
There REMAINS therefore a SABBATH rest for the people of God" Heb 4.
"
Worship HIM who MADE the heavens and the earth the seas and the springs of water" Rev 14:7
"
EVERY Sabbath" they were in the synagogue preaching the Gospel to BOTH gentiles and Jews - Acts 18:1-5.
"
EVERY Sabbath" scripture is read in the synagogues and this solves the problem of Acts 15 for gentiles.
"
The Sabbath was made for MANKIND" Mark 2:27 -- turns out --- that includes gentiles.
EVEN Dies Domini insists that the Sabbath commandment as with ALL the TEN commandments
are for ALL MANKIND and "not just jews"
Your position opposed to both the Bible and your own Pope John Paul II - is a bit 'extreme' for a Catholic don't you think??
Dies Domini pt 11
"the rest of the Sabbath..discloses something of the nuptial shape of the relationship which God wants to establish with the creature made in his image, by calling that creature to enter a pact of love".
Dies Domini pt 13 -
"the Sabbath ...is therefore rooted in the depths of God's plan. This is why unlike many other laws - it is not within the context of strictly cultic (Jewish) stipulations but within the Decalogue the "ten words" which represent the very pillars of moral life inscribed on the human heart!! In setting this commandment within the context of the basic structure of ethics, Israel and then the church declare that they consider it not just a matter of community religious discipline but a defining and indelible expression of our relationship to God, announced and expounded by biblical revelations.
Dies Domini pt 11 "if the first page of the book of Genesis presents God's work as an example for man, the same is true of God's rest - on the seventh day God finished his work which he had done therefore God blessed the seventh day and made it holy...it is a gaze which God casts upon all things, but in a special way upon man, the crown
of creation. It is a gaze which already discloses something of the nuptial shape of the relationship God wants to establish with the creature made in his own image, by calling that creature to enter a pact of love."
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This verse warns about alienating the convert.
A truly fascinating reduction and gloss-over of all the details in that post.