What is the essence of Christianity?
I am not fond of the word 'Christianity' - it speaks of a movement rather than its essence. I like the elaboration of the question that Margaret suggests in her answer "If we had to distill Christianity and the Bible down to its essential oil, what would the fragrance be?" The vines with the tender grapes give a good smell. Distilling the movement is a good idea. But the word 'Xianity' has "too many syllables".
Margaret suggests 'redemption'. My counter to this is that the Hebrew Canon is filled with redemption as an essence. Redemption does not uniquely summarize Christianity. It applies also to other faiths. To prove this I only need to point to one example that predates the Jesus Christ movement.
Redemption is the core of Ruth for example. For some within the Hebrew faith, the core might be conversion (as noted at that link) - but conversion misses the affront of the Moabite in Israel which makes illegal David's kingship (since he is within the limit of generations for the Moabite to be accepted in Israel). If I had to put the Psalms into one word, perhaps it would be 'the redeemed' if you will allow the definite article (I prefer 'the mercied' but English won't allow it without coinage - the word I am thinking of is in Psalm 149. It occurs 3 times - word 9, 32, and 62 of 64 words - so it provides the essential frame and theme.) Here again this is redemption. It is the creation of a new multitudinous class of humans that have been shown the covenant mercy and are thus given power over the political turmoil to "bind their kings in chains".
I chose 'information' in answer to Margaret's original question a month ago. In this I was following the lead of The Journey of the Magi by T. S. Eliot and his line in that poem. "But there was no information".
Then we came to a tavern with vine-leaves over the lintel,
Six hands at an open door dicing for pieces of silver,
And feet kicking the empty wine-skins.
But there was no information, and so we continued
And arrived at evening, not a moment too soon
Finding the place; it was (you may say) satisfactory.
What is new in Christ Jesus is that we who toil here (note the recurrence of the word 'toil' in Ecclesiastes) and cannot comprehend the work of God (Eccl 8:17, 11:5) are not without hope - as some might read Qohelet - (but there is hope there too, Eccl 5:19).
The Hebrew election (this is God's chosen people) informs the redemption that Christ Jesus accomplishes. The toil/work axis of Qohelet is summarized by the work that Jesus came to do (see the theme of work in both creation and redemption in John's gospel). Besides linking TNK and NT in this way (as a silly aside - the Bible is TNKNT - a chiasm with the writings in the middle), information encompasses redemption - we are formed in our inner person through the Spirit. Such inner-formation is greatly to be desired. (This pun on in-formation should not be tossed out too quickly as a slight of rhetoric.)
That's why I like information over other one-word summaries. It is unique to the Anointing that is in Christ Jesus. Other 'religions' are not without redemption, or hope, or healing, or love, but they have an incomplete language with which we might be informed. It is in this sense that Jesus completes election. (Job's completeness anticipates this fullness and the satisfactory that was to come.)
What do you think - is my claim justifiable that other religions have not got the language to express the Anointing that is in Christ Jesus, i.e. to express the incarnation of the Spirit? (I labour that Christ might be formed in you. Galatians 4:19)