You're claiming what you have yet to prove, that the Bible establishes that you were a "child of the Devil." Asserting things doesn't prove anything except that you believe something.
Well that is what you have done. At least I have working experience of actively being on Satans side of the spiritual warfare.
Yes, it is the heart that responds to God, or not. By responding positively to God's word, a person can get saved and become a "child of God." By responding negatively to God's word, a person can become a "child of Satan." The difference is the degree to which a person responds to God's word.
There are a lot of reasons that people make bad choices, which we will call "mitigating factors." This means that though a person responds badly to the word of God it does not mean that God's word is being rejected for all time. Obviously, in your case, you rejected God's word (unconsciously) but did not reject it for all time.
People make "bad" choices because we are inherently bad. As Isaiah wrote- even our rightouseness is filthy rage before God.
If you will simply read the Bible you will see how wrong you are.
John 8.44 You belong to your father, the devil, and you want to carry out your father’s desires. He was a murderer from the beginning, not holding to the truth, for there is no truth in him. When he lies, he speaks his native language, for he is a liar and the father of lies.
Acts 13.10 “You are a child of the devil and an enemy of everything that is right! You are full of all kinds of deceit and trickery. Will you never stop perverting the right ways of the Lord?"
1 John 3.10 This is how we know who the children of God are and who the children of the devil are: Anyone who does not do what is right is not God’s child, nor is anyone who does not love their brother and sister.
John 1.12 Yet to all who did receive him, to those who believed in his name, he gave the right to become children of God.
1 John 2.14 I write to you, dear children, because you know the Father. I write to you, fathers, because you know him who is from the beginning. I write to you, young men, because you are strong, and the word of God lives in you, and you have overcome the evil one.
Well this corresponds exactly to what I said in the beginning. If one is not a child of god, then by default they are a child of the devil no matter how outwardly "good" or "bad" they behave.
Even we cannot say with 100% certitu8de who the children of God are. REason?
1 John 2:19
They went out from
us, but they were
not of us; for if they had been
of us, they would no doubt have continued with
us: but they went out, that they might be made manifest that they were
not all
of us.
Matthew 7:21-23
King James Version
21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
Matthew 13:24-30
King James Version
24 Another parable put he forth unto them, saying, The kingdom of heaven is likened unto a man which sowed good seed in his field:
25 But while men slept, his enemy came and sowed tares among the wheat, and went his way.
26 But when the blade was sprung up, and brought forth fruit, then appeared the tares also.
27 So the servants of the householder came and said unto him, Sir, didst not thou sow good seed in thy field? from whence then hath it tares?
28 He said unto them, An enemy hath done this. The servants said unto him, Wilt thou then that we go and gather them up?
29 But he said, Nay; lest while ye gather up the tares, ye root up also the wheat with them.
30 Let both grow together until the harvest: and in the time of harvest I will say to the reapers, Gather ye together first the tares, and bind them in bundles to burn them: but gather the wheat into my barn.
Once again we judge on the external, but we can not know a persons heart fully.
One is either a child of God or they are a child of the devil.