It seems that belief in God is on the increase. Lots of people I meet believe in God, have faith in the existence of God, but are not children of God because their faith is not saving faith. The distinction lies in the nature of faith as Thomas Watson explains:
1 What faith is. If faith instates us into son-ship (adoption by God), it concerns us to know what faith is. There is a twofold faith.
(i) A more lax general faith. When we believe the truth of all that is revealed in the Holy Scriptures, this is not the faith which privileges us in son-ship. The devils believe all the articles in the creed. It is not the bare knowledge of a medicine or believing the sovereign virtue of it that will cure one that is ill. This general faith (so much cried up by some) will not save. Thus a man may believe and not love God. He may believe that God will come to judge the quick and the dead, and hate him, as the prisoner believes the judge’s coming to the assizes, and abhors the thoughts of him. Take heed of resting in a general faith. You may have this and be no better than devils.
(ii) There is a special faith, when we not only believe the report we hear of Christ, but rest upon him, embrace him, ‘taking hold of the horns of this altar’, resolving there to abide. …faith draws Christ into the heart and applies him there. …By this we are made the children of God, and wherever this faith is, it is not like a medicinal pill in a dead man’s mouth, but is exceedingly operative. It obliges to duty. It works by love (Galatians 5:6)