The following reflection on Genesis 15:18 comes from the handwritten notes in my ESV Journaling Bible, Interleaved Edition.
On that day the Lord made a covenant with Abram, saying, “To your offspring I give this land, from the river of Egypt to the great river, the river Euphrates...
15:18: "I give this land": Or "I have given" (NASB; see footnote in ESV). In other words, there's a sense in which God has already accomplished what he promises to do in the present. God is already in the future, after all, so there's no doubt how his promise to Abram will turn out.
Do you see how encouraging this is?
When God says that something will happen, we can be confident that it will happen this way—indeed, from God's perspective it already has happened. When he makes this promise to Abram, he has already done everything necessary to ensure that Abram's descendants have received the land; in a sense, God is already in the future, seeing that his promise is fulfilled and waiting for Abram to arrive there himself.
By contrast, when I want to assure someone that I will fulfill a promise, I might say, "Don't give it a second thought. It's done!" But it's not done. In fact, I could drop dead before I fulfill the promise, no matter how sincerely I spoke those words.
Needless to say, God's word isn't like mine: when God says, "I will do," he always means, "I have done."
Consider how this applies to the glorious promise of Romans 8:28: "And we know that for those who love God all things work together for good, for those who are called according to his purpose."
It means that the blessing waiting for us on the other side of whatever trial we happen to be going through at this moment is as sure as the stars in the sky (v. 5)