I’ve found J. I. Packer’s Growing in Christ to be a helpful and brief summary of the basics of the Christian faith: the themes of the Apostle’s Creed, the Ten Commandments, and the Lord’s Prayer. Indeed, most Reformation catechisms contain discussions of these three basics since even young children can learn and should know them. (As a side note, if you have kids these three basics should be foundational in their Christian training at home.)
I appreciate Packer’s book because each lesson is around 3 pages long and contains several application questions. I used this book for a high school level class in a church setting and it worked out pretty well. It would also be good for a small group setting or for teaching a new Christian the main aspects of Christianity.
Here’s a short excerpt from chapter fifteen, which is a discussion on the last phrase of the Lord’s Prayer.
“When in the traditional Lord’s Prayer doxology we ascribe the glory, along with the royal rule, to God forever, we are, first, telling God (and thus reminding ourselves) that he, our Maker and Redeemer, is, and always will be, glorious in all he does, especially in his acts of grace (‘we give thanks to thee for thy great glory‘); and, second, we are committing ourselves, now and always, to worship and adore him for it all (‘glory be to God on high’). The doxology thus makes the Lord’s Prayer end in praise, just as the Christian life itself will do: for while petition will cease with this life, the happy task of giving God glory will last for all eternity.”