Blasphemy against the Holy Ghost, I guess? I think gotquestions.org's position is that it cannot be done today but if I'm correct when reading Matthew and Mark I guess it seems to me like maybe anyone could do it at any time. I think gotquestions.org's position might be that since Jesus is not on the earth it is not possible to do today (along with other reasons possibly).
I agree. Got Questions is a Calvinist site. I do use them from time to time for some things but they are also anti-spiritual gifts as well
Matthew 12:32
Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the one to come.
Jesus was talking to the Pharisees
22Then a demon-possessed man who was blind and mute was brought to Jesus, and He healed the man so that he could speak and see.
23The crowds were astounded and asked, “Could this be the Son of David?”
24But when the Pharisees heard this, they said, “Only by Beelzebul, the prince of demons, does this man drive out demons.”
( the Pharisees attributed the healings and casting out of demons by Jesus as the work of the devil)
25Knowing their thoughts, Jesus said to them, “Every kingdom divided against itself will be laid waste, and every city or household divided against itself will not stand.
26If Satan drives out Satan, he is divided against himself. How then can his kingdom stand?
27And if I drive out demons by Beelzebul, by whom do your sons drive them out? So then, they will be your judges.
28But if I drive out demons by the Spirit of God, then the kingdom of God has come upon you.
29Or again, how can anyone enter a strong man’s house and steal his possessions, unless he first ties up the strong man? Then he can plunder his house.
30He who is not with Me is against Me, and he who does not gather with Me scatters.
'and then He continues and states:
31Therefore I tell you, every sin and blasphemy will be forgiven men, but the blasphemy against the Spirit will not be forgiven.
32Whoever speaks a word against the Son of Man will be forgiven, but whoever speaks against the Holy Spirit will not be forgiven, either in this age or in the one to come.
so rejecting Christ results in God not forgiving sins also, but Jesus speaks specifically about attributing to the devil that which is actually the work of the Holy Spirit
That does actually happen today