It was not enough to be a true-born Israelite, a man must be able to show his descent. The genealogical tables were kept with the greatest care; and there was a holy pride in being able to vindicate the claim of having the blood of the patriarchs in the veins.
It is a blessed thing to be sure that we have passed from death unto life, and are the children of God by faith in Christ Jesus. True, our eternal destiny does not hinge on it. Many will doubtless be saved at last, who have spent their lives between hope and fear. But it is very needful for our comfort and growth in grace to be able to declare our pedigree, and to know that we have been translated into the Kingdom of God's dear Son.
The Gospel of John was written that we might believe; the Epistle that we might know. But many seek this knowledge in the wrong way, and are exposed to endless questionings. They try to discover the date, place, or experience in the past, when they were incorporated into the Divine family; and because they cannot point to these, they imagine that they are still outside. Now for every one that has had a definite experience of the new birth, there are perhaps a score who entered the Divine family almost as a sailor passes the line of the Equator. Yet it is possible for you to know that you are born again, though you may not be able to tell your birthday.
If you are trusting Jesus, if the Spirit witnesses with your spirit that God is your Father, if you are full of a holy fear of grieving Him, if you are becoming like Him, if you love the brethren - you may certainly declare yourself His child.