What does it mean to "reason together"?

People are surprised when they learn that the following verse is not in the New Testament. It sounds New Testament-ish. But it's not in that book.

Come now, let us reason together, says the LORD: though your sins are like scarlet, they shall be as white as snow; though they are red like crimson, they shall become like wool. (Isaiah 1:18).

It's from Isaiah.

Things get even more interesting. It is the LORD God telling His people to approach Him to reason with Him, and to do it in confidence and not in fear. In the verse, God is speaking to His chosen people Israel.

Gill's Exposition explains:

Come now, and let us reason, together, saith the Lord,.... These words stand not in connection either with the preceding or following, but are to be read in a parenthesis, and are thrown in for the sake of the small remnant God had left among this wicked people, in order to comfort them, being distressed with sin. 

These, seeing their sins in their dreadful colours, and with all their aggravating circumstances, were ready to conclude that they were unpardonable; and, seeing God as an angry Judge, dared not come nigh him, but stood at a distance, fearing and expecting his vengeance to fall upon them, and therefore put away the promises, and refused to be comforted; 

when the Lord was pleased to encourage them to draw near to him, and come and reason with him: not at the bar of his justice; there is no reasoning with him there; none can contend with him, or answer him, one of a thousand; if he marks iniquity in strict justice, none can stand before him; there is no entering the lists with him upon the foot of justice, or at its bar: 

but at the bar of mercy, at the throne of grace; there the righteous may dispute with him from his declarations and promises, as well as come with boldness to him; and at the altar and sacrifice of Christ, and at the fountain of his blood: 

here sinners may reason with him from the virtue and efficacy of his blood and sacrifice; and from the Lord's proclamation of grace and mercy through him; and from his promises to forgive repenting and confessing sinners: and here God reasons with sensible souls from his own covenant promises and proclamations to forgive sin; from the aboundings of his grace over abounding sin; from the righteousness of Christ to justify, his blood to cleanse from sin, and his sacrifice to atone for it.

We have a good God, abounding in mercy and patience. His grace is eternal. His Son's atoning sacrifice is eternal. Our communion and reasoning with Him is eternal.