לָכֵ֞ן אֱמֹ֥ר לִבְנֵֽי־יִשְׂרָאֵ֘ל אֲנִ֣י יְהוָה֒ וְהוֹצֵאתִ֣י אֶתְכֶ֗ם מִתַּ֙חַת֙ סִבְלֹ֣ת מִצְרַ֔יִם וְהִצַּלְתִּ֥י אֶתְכֶ֖ם מֵעֲבֹדָתָ֑ם וְגָאַלְתִּ֤י אֶתְכֶם֙ בִּזְר֣וֹעַ נְטוּיָ֔ה וּבִשְׁפָטִ֖ים גְּדֹלִֽים׃
Rashi on Exodus
לכן THEREFORE, i. e. in accordance with the tenor of that oath (cf. Exodus Rabbah 6)
Ramban on Exodus
AND I WILL BRING YOU OUT FROM UNDER THE BURDENS OF THE EGYPTIANS. He assured them that He will take them out from the land of the Egyptians and that they will no longer suffer from their heavy burden.
Sforno on Exodus
'לכן אמור לבני ישראל אני ה. The Torah speaks of three reasons (stages of the redemption) why G’d at this time reveals knowledge about Himself which He had not revealed previously.