This question can be approached from the vantages of various religion traditions, but for the sake of specificity, I will consider it from an orthodox Christian standpoint. One could say that God gives us assurance today by providing us with the confidence that comes from knowing that we are always loved by Him, no matter what.
The world today, as it always has been, can be a very difficult place in which to live. War, crime, disease, famine, and all manner of evils seem to beset us on all sides. Under such trying conditions, it's all too easy to succumb to despair and lose hope. It's also much easier than it used to be, with the internet now a daily part of most people's lives, to lose oneself in all kinds of meaningless diversions and distractions.
Although we may live in a harsh, frightening world, a world subject to radical changes that we often find hard to deal with, we can, if we subscribe to the teachings of Christianity, be assured that God and His love for us remain as strong and unchanging as ever. The knowledge of this provides us with certainty in an uncertain world and acts as the still point in a world that never stops turning.
If we recognize the unchanging love of God and strive to emulate it through our interactions with others, then we will have a firm foundation on which to build our lives. Among other things, this will give us much more assurance, irrespective of how much the world may change and how difficult it can be to deal with such change.