"In One" is sung from Mary's point of view as she sees the perfection of God's work and love in this one beautiful child. I expect that feeling is fairly universal among new mothers. How could you hold and look at a newborn child and not ponder the miracle and perfection of it all? It, naturally, has a larger meaning when you're holding and looking at the Christ child.
If you're digging "In One", you might be interested to know that Tanya Godsey also has a sampler EP on Noisetrade. Same terms apply, with all tips going to Doctors Without Borders. And she has a 2013 Christmas single--"O Holy Night"--available through Bandcamp (that one will cost you a dollar). Look for a new album from Tanya in 2016 and stay up to date on that via Facebook.