Q. Have you used any of these study guides with people who did not consider themselves Christians? What reaction did they have to the material?
I’ve personally used the guides in several groups that included people who didn’t consider themselves Christians. They appeared to feel very much at home and they participated freely. In these groups we took turns reading the various discussion points and asking the questions related to them, and even though I said that anyone who didn’t want to do this could “pass,” our not-yet-Christian friends were always happy to take their turns.
I think they were so comfortable because the study guides are intentionally written in such a way that people who aren’t yet followers of Jesus feel welcome and included. Because the guides invite people to engage the Bible through the lens of their own experience, everyone has something to share in response to the discussion questions, even if they don’t have a lot of biblical background or doctrinal knowledge.
The questions themselves often begin with qualifiers such as “If you are a follower of Jesus” or “If you’re part of a community of Jesus’ followers,” and they provide other options for people who don’t fit these descriptions. (For example, “If you’re not part of a community of Jesus’ followers, talk about one you’ve visited or heard about.”)
Guides often invite people to share where they are along their spiritual journeys. For 
example, the guide to Paul’s Journey Letters asks (in Session 24), “Before you began this study of Paul’s letters, where would you have put yourself on a scale of 1 to 10, where 1 is not believing in Jesus, 5 is the threshold of believing, and 10 is a settled, unreserved faith and trust in Jesus for life? Where would you put yourself now? If you’ve ‘stepped across the threshold,” or if you want to know more about how you can do that, share this with the group and ask them to talk and pray with you about it.”
This question is typical of the ones, found in other guides as well, in which people are asked how they’ve moved along in their spiritual journeys in the course of the study. There’s typically an opportunity in one or more sessions in each guide for people who are ready to become followers of Jesus to make that commitment with the group’s help.
So I think your friends who aren’t yet followers of Jesus would feel welcome, encouraged, and also graciously invited towards faith in a group that was using one of these guides. (What I’d love to see some day is a group made up predominantly of not-yet-Christians using the guides, as I think the discussions in such a group would be unpredictable and dynamic. If you put a group like that together, please invite me to visit!)
Good Question 