One is only a child of God if one is walking in righteousness according to the leading of the Spirit.
Jesus clearly taught...
Joh 8:39 They answered and said unto him, Abraham is our father. Jesus saith unto them, If ye were Abraham's children, ye would do the works of Abraham.
Joh 8:40 But now ye seek to kill me, a man that hath told you the truth, which I have heard of God: this did not Abraham.
Joh 8:41 Ye do the deeds of your father. Then said they to him, We be not born of fornication; we have one Father, even God.
Joh 8:42 Jesus said unto them, If God were your Father, ye would love me: for I proceeded forth and came from God; neither came I of myself, but he sent me.
Joh 8:43 Why do ye not understand my speech? even because ye cannot hear my word.
Joh 8:44 Ye are of your father the devil, and the lusts of your father ye will do. He was a murderer from the beginning, and abode not in the truth, because there is no truth in him. When he speaketh a lie, he speaketh of his own: for he is a liar, and the father of it.
It is erroneous to believe that one can willfully engage in wickedness and be a true child of God.
He who sins is the servant of sin is what Jesus taught. Why? Because willful sinning is an act of the will and is an act of defiance to God. To willfully sin is to choose evil over good and the children of God simply do not choose evil over God because such a thing is abhorrent.
Yet in 2015 we are faced with a religion which poses as Christianity in which its adherents believe they can willfully choose to engage in evil and yet still claim to be children of God. These people believe in a salvation which leaves people wicked. What kind of salvation is that? What do these people think they are saved from what? Certainly not saved from sin and wickedness.
Salvation to them is merely a salvation from judgement not inward wickedness. Thus the inward wickedness still exists, yet because they perceive they reap no condemnation for it, clearly, the salvation they believe, is a license to engage in wickedness without condemnation.
What is the different between the Christian and the non-Christian to these people? The only real difference is that they think that their wickedness is covered by some provision they believe took place on the cross. Thus the Gospel to them is a cloak for wickedness once all the fluff is stripped away.
The children of God are born again of the Spirit and are inwardly transformed. The inward transformation necessitates outward holiness. Anyone walking according to the lusts of the flesh in rebellion to God and yet believes they are a child of God because they "believe in Jesus" is extremely deceived.