Jared Wilson, from his excellent book call Gospel Deeps, writes:
I have heard it said that the gospel is shallow enough that it is safe for a toddler to swim in, yet deep enough to drown an elephant. We might also think of it this way: We teach our little ones how to read by first teaching them their ABCs. From there, they may move on to the basic principles of phonics. ABCs and phonics are scaled for little children to grasp the English language. But some people get advanced degrees in linguistics. Same category, different levels. The gospel is like that. The ABCs of the gospel work very well for people at all levels of their faith, including wise and old pastors and brilliant theologians, but it’s possible to explore the ABCs into their inherent complexity.
Although a small child can learn the basics of the English language, many people will nevertheless tell you that English is not the easiest of languages to learn. In the same way, even the simple gospel can be seen less simply. Suppose we use the template “God saves sinners through the life, death, and resurrection of Jesus Christ”. We could go on point by point through that simple statement and find depth along the way. God saves sinners through Jesus’ life? How so? Suddenly we are talking about Christ’s active obedience, the tension of the incarnation, the reality of temptation and the reality of sinlessness, and the like. How does God save sinners through Jesus’ death? There is a wealth of truth there, and now we are on the verge of discussing the various theories of the atonement. And since the resurrection changes everything, we are ready to talk about everything when we get to it! What sort of salvation does Jesus’ resurrection enact for sinners?
What we are glimpsing now is how a wardrobe can contain a world.