As soon as we clearly see what the kingdom of God is, we see also that by nature we do not belong to it. The kingdom of God so far as man is concerned is a state of willing subjection to Him--a state in which we are in our right relation to Him. All irrational creatures obey God and do His will: the sun runs his course with an exactness and punctuality we cannot rival; the grace and strength of many of the lower animals, their marvellous instincts and aptitudes, are so superior to anything in ourselves that we cannot even comprehend them. But what we have as our speciality is to render to God a willing service; to understand His purposes and enter sympathetically into them. The lower creatures obey a law impressed upon their nature; they cannot sin; their performance of God's will is a tribute to the power which made them so skilfully, but it lacks all conscious recognition of His worthiness to be served and all knowledge of His object in creation. It is God serving Himself: He made them so, and therefore they do His will. So it is with men who merely obey their nature: they may do kindly, noble, heroic actions, but they lack all reference to God; and however excellent these actions are, they give no guarantee that the men who do them would sympathize with God in all things, and do His will gladly.
Indeed, to establish the proposition that flesh or nature does not give us entrance into God's kingdom, we need go no further than our own consciousness. Remove the restraints which grace puts upon our nature, and we are aware that we are not in sympathy with God, fond of His will, disposed for His service. Let nature have its swing, and every man knows it is not the kingdom of God it takes him to. To all men it is natural to eat, drink, sleep, think; we are born to these things, and need to put no constraint on our nature to do them; but can any man say it has come naturally to him to be what he ought to be to God? Do we not to this hour feel drawn away from God as if we were not in our element in His presence? Flesh, nature, in God's presence is as much out of its element as a stone in the air or a fish out of water. Men who have had the deepest religious experience have seen it most clearly, and have felt, like Paul, that the flesh lusts against the spirit, and draws us ever back from entire submission to God and delight in Him.
Perhaps the necessity of the second birth may be more clearly apprehended if we consider it from another point of view. In this world we find a number of creatures which have what is known as animal life. They can work, and feel, and, in a fashion, think. They have wills, and certain dispositions, and distinctive characteristics. Every creature that has animal life has a certain nature according to its kind, and determined by its parentage; and this nature which the animal receives from its parents determines from the first the capabilities and sphere of the animal's life. The mole cannot soar in the face of the sun like the eagle; neither can the bird that comes out of the eagle's egg burrow like the mole. No training can possibly make the tortoise as swift as the antelope, or the antelope as strong as the lion. If a mole began to fly and enjoy the sunlight it must be counted a new kind of creature, and no longer a mole. The very fact of its passing certain limitations shows that another nature has somehow been infused into it. Beyond its own nature no animal can act. You might as well attempt to give the eagle the appearance of the serpent as try to teach it to crawl. Each kind of animal is by its birth endowed with its own nature, fitting it to do certain things, and making other things impossible. So is it with us: we are born with certain faculties and endowments, with a certain nature; and just as all animals, without receiving any new, individual, supernatural help from God, can act according to their nature, so can we. We, being human, have a high and richly-endowed animal nature, a nature that leads us not only to eat, drink, sleep, and fight like the lower animals, but a nature which leads us to think and to love, and which, by culture and education, can enjoy a much richer and wider life than the lower creatures. Men need not be in the kingdom of God in order to do much that is admirable, noble, lovely, because their nature as animals fits them for that. If we were to exist at all as a race of animals superior to all others, then all this is just what must be found in us. Irrespective of any kingdom of God at all, irrespective of any knowledge of God or reference to Him, we have a life in this world, and a nature fitting us for it. And it is this we have by our natural birth, a place among our kind, an animal life. The first man, from whom we all descend, was, as St. Paul profoundly says, "a living soul," that is to say, an animal, a living human being; but he had not "a quickening spirit," could not give to his children spiritual life and make them children of God.