Complex sentence: According to this idea, Sukkot is a universal holiday, but Shemini Atzeret, is only for the Jewish people, and it is a modest holiday just to celebrate (God's) special relationship with His beloved nation.
Split into simpler sentences: According to this idea, Sukkot is a universal holiday, but Shemini Atzeret is only for the Jewish people. Moreover, Shemini Atzeret is a modest holiday, just to celebrate (God's) special relationship with His beloved nation.