In today's gospels the leaders who questioned Jesus, when he in turn questions them about the origins of John the Baptist's authority, answer simply " We don't know." Had that been a truthful answer, there would have been no sin in that. But the gospel tells us they said they did not know, not because they were truly ignorant but because they were afraid of the answers yes and no. Rather than acknowledge the truth they know in their heart, they pretend to not know.
The conscience is a marvelous thing, we each have the basic law of God hard-wired into us. But from toddlerhood we learn to be like those leaders. How old were any of us the first time we did something bad and when Mom asked, "Why did you do that?" We sheepishly responded, " I don't know." In fact the toddler may not be able to articulate why they did it but they knew it was wrong.
Who of us can drive past the poor man on the street corner and not feel the tug of our conscience? And I've never met the cohabitating couple who didn't show some sign of shame. We can pretend and at times even lie to ourselves, but the conscience knows the truth. We know sin when we commit it.
One caveat on proclaiming the truth. Start with yourself! Often we are far too ready to name someone else's sin. Here the words of St. Matthew are the best guide:
first take the plank out of your own eye, and then you will see clearly to remove the speck from your brother's eye.