In my mind, the worst sin that any human being could ever, would ever, or has ever committed is very clear. The worst possible sin would be to kill the Son of God. I can’t imagine anything worse than that; it not only embodies murder, but the most hateful, venomous, vicious rejection of God. Yet it is precisely that sin which Jesus demonstrates is forgivable. In Luke 23:34, as He hangs on the cross, He looks down at those who have taken His life, then He looks to the Father and says, “Father, forgive them.” Even killing the Son of God is forgivable. It isn’t the volume of sin that is unforgivable any more than it is the kind of sin.
John MacArthur

Bible in One Year