Here the Almighty gives us permission,even more, .asks us, to test Him[בְחָנוּנִי נָא בָּזֹאת,[[ try Me now herewith]

Therefore it is a sign of absolute emuna that by giving as the Tora recommends/demands from us we wll not loose anything but are promised by the word of the Almighty Himself that he will pour out His blessing on us [אֶפְתַּח לָכֶם, אֵת אֲרֻבּוֹת הַשָּׁמַיִם, וַהֲרִיקֹתִי לָכֶם בְּרָכָה, עַד-בְּלִי-דָי ]

י  הָבִיאוּ אֶת-כָּל-הַמַּעֲשֵׂר אֶל-בֵּית הָאוֹצָר, וִיהִי טֶרֶף בְּבֵיתִי, וּבְחָנוּנִי נָא בָּזֹאת, אָמַר יְהוָה צְבָאוֹת:  אִם-לֹא אֶפְתַּח לָכֶם, אֵת אֲרֻבּוֹת הַשָּׁמַיִם, וַהֲרִיקֹתִי לָכֶם בְּרָכָה, עַד-בְּלִי-דָי.10 Bring ye the whole tithe into the store-house, that there may be food in My house, and try Me now herewith, saith the LORD of hosts, if I will not open you the windows of heaven, and pour you out a blessing, that there shall be more than sufficiency.
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