Since Reformed doctrine is regeneration, then belief, let's see what John says about some who believed in Jesus.
John 8 -
30 As he spake these words, many believed on him.
31 Then said Jesus to those Jews which believed on him, if ye continue in my word, then are ye my disciples indeed;
We see many believed on Jesus, which according to Reformed doctrine, had to be regenerated first. Now let's see how these regenerated Jews act toward Jesus.
32 And ye shall know the truth, and the truth shall make you free.
33 They answered him, We be Abraham's seed, and were never in bondage to any man: how sayest thou, Ye shall be made free?
34 Jesus answered them, Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin.
35 And the servant abideth not in the house for ever: but the Son abideth ever.
36 If the Son therefore shall make you free, ye shall be free indeed.
37 I know that ye are Abraham's seed; but ye seek to kill me, because my word hath no place in you.
Jesus told the 'regenerated' Jews that they sought to kill Him, because His word had no place in them. So we have Jews with a 'new heart' seeking to kill Jesus.
38 I speak that which I have seen with my Father: and ye do that which ye have seen with your father.
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.
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.
Jesus said these 'regenerated' Jews sought to kill Him the second time, even when Jesus told them the truth.
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.
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.
43 Why do ye not understand my speech? even because ye cannot hear my word.
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.
Here's the real kicker - Jesus said these 'regenerated' Jews with a 'new heart' are of their father, the devil. I'm not seeing regeneration before belief in these supposed 'regenerated' Jews. But some want to believe this, despite what we read here.
45 And because I tell you the truth, ye believe me not.
Now Jesus tells these 'regenerated' Jews, they do not believe Him, because Jesus told them the truth. How can one be regenerated and not believe Jesus?
46 Which of you convinceth me of sin? And if I say the truth, why do ye not believe me?
47 He that is of God heareth God's words: ye therefore hear them not, because ye are not of God.
More condemning words from the Savior. He tells these 'regenerated and believing' Jews, they do not hear His words, because they are not of God.
48 Then answered the Jews, and said unto him, Say we not well that thou art a Samaritan, and hast a devil?
49 Jesus answered, I have not a devil; but I honour my Father, and ye do dishonour me.
Is anyone seeing the fault with regeneration before belief? These supposed 'regenerated and believing' Jews now tell Jesus He has a devil.
50 And I seek not mine own glory: there is one that seeketh and judgeth.
51 Verily, verily, I say unto you, If a man keep my saying, he shall never see death.
52 Then said the Jews unto him, Now we know that thou hast a devil. Abraham is dead, and the prophets; and thou sayest, If a man keep my saying, he shall never taste of death.
53 Art thou greater than our father Abraham, which is dead? and the prophets are dead: whom makest thou thyself?
54 Jesus answered, If I honour myself, my honour is nothing: it is my Father that honoureth me; of whom ye say, that he is your God:
55 Yet ye have not known him; but I know him: and if I should say, I know him not, I shall be a liar like unto you: but I know him, and keep his saying.
Jesus now tells them they have not known God, but the Reformed want us to believe they have been regenerated by God.
56 Your father Abraham rejoiced to see my day: and he saw it, and was glad.
57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham?
58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
59 Then took they up stones to cast at him: but Jesus hid himself, and went out of the temple, going through the midst of them, and so passed by.
Finally, these so-called 'regenerated and believing' Jews are so fed up with the words of Jesus, they take up stones to throw at Him. Is this what you should believe about regenerated people?
It's your life, choose carefully and wisely who and what you believe. Deception cares not who it takes under.
Ask and seek what true regeneration is. I can honestly say, if your father is the devil, and the words of Jesus have no place in you, even though you say you believe, YOU ARE NOT REGENERATED.