Hashem provides the detailed instructions, the resources, the means of putting it all together through his gift of the Spirit upon Betzalel the master-builder. But everything hinges on the willing, generous hearts of the Israelites. They must freely offer up all the materials for the Mishkan, the rich dyed fabrics, the gold, silver, and copper, the precious stones and the fragrant spices for the incense and anointing oil. In the midst of all this abundance the one indispensable factor is nadiv lev, the generous heart of the Israelites who must bring it all together (Ex. 25:2; 35:5, 21, 29; 36:2).
Exodus 35:5 distills this whole theme in one verse: "Take from among you an offering to Hashem--terumah l'Adonai. Whoever is of a generous heart let him bring terumat Adonai--Hashem's offering." Note the change of language here, which joins the two words, terumah and Adonai, in what is called in Hebrew smikhut or the construct state, combining two nouns to create one new one. The generous heart transforms an offering to the Lord--terumah l'Adonai--into the-Lord's-offering--terumat Adonai. It is the wholehearted response, the middah of responsibility described as nadiv lev, that transforms something we bring to the Lord into something that belongs to the Lord himself and is linked to his own name.