Jesus said, "Show me the stone that the builders rejected: that is the keystone." (66 Thomas)

What is rejected will become what is most essential. What others (most?) considered insufficient will become what is essential. What seems bad will become very good.

This aphorism also follows all of the synoptic accounts of the parable of the vineyard. In Mark, Matthew, and Luke the saying is used to explain the parable. In this way we are - or thousands of sermons have - led us to understand that the rejected Jesus is the keystone.

But to me it seems likely Jesus was referencing Psalm 118:22, here in context:

כ זֶה-הַשַּׁעַר לַיהוָה; צַדִּיקִים, יָבֹאוּ בוֹ.
This is the gate of the LORD; the righteous shall enter into it.
כא אוֹדְךָ, כִּי עֲנִיתָנִי; וַתְּהִי-לִי, לִישׁוּעָה.
I will give thanks unto Thee, for Thou hast answered me, and art become my salvation.
כב אֶבֶן, מָאֲסוּ הַבּוֹנִים-- הָיְתָה, לְרֹאשׁ פִּנָּה.
The stone which the builders rejected is become the chief corner-stone.
כג מֵאֵת יְהוָה, הָיְתָה זֹּאת; הִיא נִפְלָאת בְּעֵינֵינוּ.
This is the LORD'S doing; it is marvellous in our eyes.
כד זֶה-הַיּוֹם, עָשָׂה יְהוָה; נָגִילָה וְנִשְׂמְחָה בוֹ.
This is the day which the LORD hath made; we will rejoice and be glad in it.

In the psalm the gate is our troubles, our "straits" in the King James version. All the challenges, troubles, and hurts... all that I am inclined to reject... can bring us through the wall that separates us from fulfillment, completeness, wholeness.

If we will love and honor Reality - especially at its most difficult - there we can find purpose and we will rejoice and be glad in it.