It's challenging to “get” certain spiritual concepts—like the Trinity—but that's where Mary Poppins can help.
Yes, Mary Poppins—the movie.
There's a scene early on in her new room at the Banks' house. As she's looking in the mirror and putting on her hat, she (naturally) starts singing. Then her image in the mirror begins to move on its own and sing with her, harmonizing and complementing her song. Then the image goes off into a high, flamboyant aria. The real Mary Poppins snorts, “Cheeky!” and stomps off.
You can see immediately how this relates to the Trinity, can't you?
...Okay, let me elaborate.
The Son is the perfect image of the Father, like the reflection in the mirror is an exact image of Mary Poppins. Mary—being magic—gives her image the ability to think and move independently. The Son too is not merely an unthinking reflection but another Person, begotten by the Father.
Okay, so what about the Holy Spirit? There's only two persons in this analogy.
Well—as usual—the Spirit is felt rather than seen here. But to be more precise—in Mary Poppins' case, the “spirit” is not felt, or at least is felt to be missing. Conspicuous by its absence.
The Spirit of the Trinity is much more evident. The Son is not cheeky; instead, He does “whatever [He] see[s] [His] Father doing”. Unlike the magical mirror image—who didn't care about the wishes of her source, but wanted to show off and glorify herself—Jesus thinks and wills like His Father: they have the same “spirit”.
This part of the Trinity isn't really as foreign as we might think at first; the idea is part of our language. When students love their school and cheer for their team, what do they show? School spirit. When a group has the same passion and works together, what do they have? One spirit.
We are made in the image of God, how does that fit in? Well, we can apply this analogy to ourselves too, remembering though that's not as apropos. Jesus is the “only-begotten Son of the Father”, after all. He's a perfect image, a life-size image, with all the powers of His Father. Let's just say we're “mini-images”.
But we too are free to do as we like, and called to freely do whatever our Father does. We don't see the Father though, as Jesus did. So we must imitate the Son. And that's only right for another reason: we have been baptized into Christ; we are His Body. That's how we get to heaven, not on our own merits, but by being one with Christ.
That's where the phrase “WWJD?” comes in handy. But it can't merely be an occasional nice thought. Christ's life must be replicated in mine. I must crucify my self-will and die to myself to live in Him.
But He doesn't leave us orphan, but promised at His Ascension to be with us always. How He does so is reflected in this cluster of feasts: next week's, Corpus Christi, reminds us that in the Eucharist, He is physically with us—and when we receive, in us. And last week's feast of Pentecost celebrates the fulfillment of His promise to send us His Spirit.
It's up to us not to cut ourselves off from Him, but to stay connected to Him, as close as a member of His body: to do as we see Him doing, so that we might share one Spirit with Him.