Day 15 of 40 is all abouit how God was David's help.

"The Chronicler is writing to the postexilic Jewish community to assure them them that even on the far side of the exile, God's faithfulness to David continues. God's covenant with David contains two essential components: the perpetuity of David's line and kingdom, and the choice of Jerusalem as the site where God will place His name." (Dr. Timothy Berrey, "40 Days Through The Bible")