From a purely artistic view, no, you can have a fictional Jesus Christ say whatever you want. There's no reason for a straw-man named Jesus to be deep, and Lord knows there have been enough straw-man Jesus characterizations throughout the year that are far more offensive to Christians than just some conversations in a sci-fi story.
(Note that, depending on how hard your sci-fi is, a Crystal Dragon Jesus may be a nice way to sidestep the entire issue.)
If you are trying to not be offensive and hew to the image that most Christians have of our messiah, you should be able to fit Him into the story easily enough. I'd offer the following guidelines:
- Decide early on if you're going to have "Jesus Christ the Christian Deity" or "historical human named Jesus but God doesn't exist", or be purposefully opaque. All are acceptable choices, but you should be consistent.
- Jesus as depcited in the gospels was not a fire-and-brimstone, old-testament preacher. He was the guy who preached the sermon on the mount ("Love God as yourself, Love thy neighbor") and stood up for a prostitute who would have been stoned to death ("Let he who is without sin cast the first stone.")
- Jesus did get angry in a few cases, most famously when he kicked the money-changers out of the temple.
- As you noted in your question, Jesus as depicted in the gospels was given to making very profound statements. He taught largely through some profound pronouncements and a slew of parables, and at once expected more of men that they could achieve and yet accepted them as they failed.
When it comes to just answering some questions from a non-belieiver, your characterization of Christ will be driven very largely by the topic of your story. If it's a tract for Agnosticism, or Christianity, or Atheism, you'll undoutably wind up treading on someone's toes. If it's more of a speculation story about alien life and space travel and how they'd fit into Christian thought, anything from "they are already christians" to "aliens are demons" would be plausible.
