D&C 88

I've always felt the 88th section of the Doctrine and Covenants was one of the greater revelations we received of the Lord.  In this revelation the Lord said -

"...I give unto you, who are the first laborers in this last kingdom, a commandment that you assemble yourselves together, and organize yourselves, and prepare yourselves, and sanctify yourselves; yea, purify your hearts, and cleanse your hands and your feet before me, that I may make you clean..."  (Doctrine and Covenants Section 88:74)

It was important that these "first laborers" become clean to accomplish the purposes of God so -

"That I may testify unto your Father, and your God, and my God, that you are clean from the blood of this wicked generation; that I may fulfil this promise, this great and last promise, which I have made unto you, when I will."  (Doctrine and Covenants Section 88:75)

The condition for this last great promise to fulfilled would have to be based upon their cleanliness.  I sense that to become clean required His saints to assemble and to organize.  I consider organize would mean to come to one unified body or -

"...unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ..."  (New Testament Ephesians 4:13)

Which means as we prepare and sanctify ourselve we come unto "the measure of the stature of the fulness of Christ".  The Lord doesn't necessarily say whag the promise is.  But what I believe He is referring to is the restitution of all things or the last dispensation of keys necessary for salvation which took place sometime later in the Kirtland Temple.