It seems to me (from reading this pdf
http://www.existentialchristianity.net/ECexistentialcommunication.pdf ) that the difference is that existential Christianity is about doing rather than knowing. Or that it is about both but that doing is more important than knowing. My conclusions may be wrong so maybe someone smarter than me can read it and comment. It is an interesting subject.
''Since God forgives on an existential basis, and we show love to God existentially, it is apparent that the practical receives a much higher place in Christianity than the theoretical. The purpose of Existential Christianity, however, is not merely to privilege the practical and subordinate the theoretical. Instead, it answers the fundamental questions of life, religion, and theology in the existential level of our existence. Questions like “How do we attain forgiveness from God?”, “How do show God that we love him?”, “What is the important parts of Christianity and what isn’t?” can all be answered with the existential both truthfully and scripturally. The essence of Christianity lies within our existence in the world, and not the intellectual contemplation within our mind. In Conclusion, Christianity is an existential communication. It is expressed to others existentially, and is expressed to God existentially. The measure of a Christian is not piety or theological understanding, but rather his imitation of Christ''
Edit: Silly me, now that I went to the site the pdf came from I see this so I think my conclusions were basically correct.
Existential Christianity
''My name is Timothy Neal, and I have established this website in order to explore the idea that Christianity is an existential communication. The existential has been ignored in Christianity for centuries, and it is my hope to continue the work of Soren Kierkegaard and otherand quotes Christian Existentialists in locating the essence of Christianity in our existence within the world.
Thus, in aldn Existential Christianity; doctrine, dogma, and systematic theology becomes sub-ordinated. The existential ideals that Jesus espoused should be the core of any Christian philosophy, and the true way to follow Jesus.''