a. What is God's judgment against sinners based on?
Answer: God's judgment against sinners is based on TRUTH, (verse 2).
MORE INFORMATION AND/OR OTHER SCRIPTURE REFERENCES:
"Truth" is Jesus Christ:
John 14:6-7: "Jesus answered, ‘I am the Way and the Truth and the Life. No one comes to the Father except through Me. If you really knew Me, you would know My Father as well. From now on, you do know Him and have seen Him.'"
Jesus is the Word:
John 1:1: "In the beginning was the Word, and the Word was with God, and the Word was God."
The Words of Jesus will judge us because they are the Words the Father told Him to say:
John 8:28: "So Jesus said, ‘When you have lifted up the Son of Man, then you will know that I am the One I claim to be and that I do nothing on My Own but speak just what the Father has taught Me.'"
John 12:49-50: [Jesus said,] "For I did not speak of My Own accord, but the Father Who sent Me commanded Me what to say and how to say it. I know that His command leads to eternal life. So whatever I say is just what the Father has told Me to say."
God has given Jesus, the Son of God, authority to judge because He is also the Son of Man, having lived among us, being tempted just like us, and yet living without sin:
John 1:14: "The Word became flesh and made His dwelling among us. We have seen His glory, the glory of the One and Only, Who came from the Father, full of grace and Truth."
And so, God's judgment against sinners is based on Truth--Jesus Christ, the Word of God:
In John 5:24-27, Jesus Himself says, "I tell you the Truth, whoever hears My Word and believes Him Who sent Me has eternal life and will not be condemned; he has crossed over from death to life. I tell you the Truth, a time is coming and has now come when the dead will hear the voice of the Son of God and those who hear will live. For as the Father has life in Himself, so He has granted the Son to have life in Himself. And He [God] has given Him [Jesus] authority to judge because He is the Son of Man. Do not be amazed at this, for a time is coming when all who are in their graves will hear His voice and come out--those who have done good will rise to live, and those who have done evil will rise to be condemned. By Myself I can do nothing; I judge only as I hear, and My judgment is just, for I seek not to please Myself but Him Who sent Me."