Just because I have life this moment does not mean that I am guaranteed life in any future moment.
For the believer, this is a comforting statement because he can be happy with life here and he is happy with life in eternity (which is Paul’s point in Phil. 1).
For the unbeliever, this is a most distressing statement because his greatest possession he will ever have is life here. The believer is certain that the best that he experiences on earth is far less and worse than he will experience in the perfections of heaven. Conversely, the worst that the unbeliever experiences on earth will be far better than the horrors of eternal hell.
So it is that the uncertainties of the continuance of life should stimulate great reflection and hesitation on the sinners continued path of rejecting Christ. To lose life is to lose everything and he has no means of guaranteeing even one more breath:
It is no security to wicked men for one moment, that there are no visible means of death at hand. It is no security to a natural man, that he is now in health, and that he does not see which way he should now immediately go out of the world by any accident, and that there is no visible danger in any respect in his circumstances. The manifold and continual experience of the world in all ages, shows this is no evidence, that a man is not on the very brink of eternity, and that the next step will not be into another world. The unseen, unthought-of ways and means of persons going suddenly out of the world are innumerable and inconceivable. Unconverted men walk over the pit of hell on a rotten covering, and there are innumerable places in this covering so weak that they will not bear their weight, and these places are not seen. The arrows of death fly unseen at noon-day; the sharpest sight cannot discern them. [Jonathan Edwards]