"Faith is the assurance of things hoped for . . . ." (Heb. 11:1). But what do I hope for? I spend a lot of time hoping that things will change for the better -- things of this world, e.g. my relationship with John as he grows, or our finances or whatever. But I get in trouble spiritually when I put my faith in what I hope to happen today: that dinner will be good, that John will nap well, that I'll have a good day. To take a verse slightly out of context, "If in this life only we have hoped in Christ, we are of all people most to be pitied. " (1 Cor. 15:29).
But if I hope in things that are eternal -- God's faithfulness and goodness, heaven, my relationship with God through Jesus' death, freedom from sin and death -- then one day my hope will be fulfilled, and so this kind of faith is true and warranted. And of course it is this kind of faith that believers are commended for: again, in Hebrews 11, even though they "did not receive what was promised" (Heb. 11:39) . . . yet.