וְלָקַחְתִּ֨י אֶתְכֶ֥ם לִי֙ לְעָ֔ם וְהָיִ֥יתִי לָכֶ֖ם לֵֽאלֹהִ֑ים וִֽידַעְתֶּ֗ם כִּ֣י אֲנִ֤י יְהוָה֙ אֱלֹ֣הֵיכֶ֔ם הַמּוֹצִ֣יא אֶתְכֶ֔ם מִתַּ֖חַת סִבְל֥וֹת מִצְרָֽיִם׃
Je vous adopterai pour peuple, je deviendrai votre Dieu; et vous reconnaîtrez que moi, l’Éternel, je suis votre Dieu, moi qui vous aurai soustraits aux tribulations de l’Égypte.
Ramban on Exodus
AND I WILL TAKE YOU TO ME FOR A PEOPLE. That is, when you will come to Mount Sinai and you will accept the Torah. There, [at Mount Sinai], it was said, Then ye shall be Mine own treasure.41Further, 19:5.
Sforno on Exodus
ולקחתי אתכם לי לעם, this will occur at the revelation at Mount Sinai.
Or HaChaim on Exodus
וידעתם כי אני ה׳ אלוקיכם, "you will know that I י־ה־ו־ה am the Lord your G'd." G'd means that the Israelites will now receive clear evidence which attribute of G'd it is that will set in motion the various steps of the redemption. They will also realise in retrospect that the attribute of Mercy had been predominant in all of G'd's actions.