Rashi on Exodus 19:5/רש"י על שמות י״ט:ה׳

1ועתה AND NOW — if you will now take upon yourselves the observance of My commandments, it will be pleasant (easy) to you from now and henceforth, for every beginning is difficult (Mekhilta).

2ושמרתם את בריתי AND KEEP MY COVENANT which I shall make with you regarding the observance of the Torah (cf. (Mekhilta).

3סגלה means a cherished treasure, the same as (Ecclesiastes 2:8) “and treasures (וסגלת) of kings” — costly vessels and precious stones which kings store up. In the same manner shall ye be unto Me a cherished treasure more than other peoples (Mekhilta) . Now do not say that ye alone belong to Me and that I have no other peoples together with (besides) you, and what else, therefore, have I by which the special love I bear you can be made evident; this is not so,

4כי לי כל הארץ FOR ALL THE EARTH IS MINE, but in My eyes and before Me they are as nought.