Summary: The second is the evidence of an inner voice or conscience that instinctively knows the difference between right and wrong, a moral compass, that none of the rest of His creation has.
God gave you and me a Conscience, why?
God wants every human being to enter and live forever in His kingdom. However His kingdom is one of righteousness meaning there is no sin there, that is, there is never any disobedience to God there. Because Adam and Eve chose to believe Satan’s words instead of God’s words, a sin nature, the inclination to disobey God, entered the human race and has infected us all. The primary purpose of Jesus Christ, the righteous Son of God, becoming a member of the human race was and is to free each of us, if we will allow Him to, from both sin’s penalty, and the power over us of our sin nature.
In order for this to happen we each must come to know God personally by believing the revealed words of the gospel that He brings to us by His Spirit. First, to receive His forgiveness of our actual sins, if we repent of them, because Christ bore those and was judged for them on the cross. Second, to receive His deliverance from the control of our sin nature or flesh, because Christ died to that sin nature on our behalf on the cross. By these two principal parts of salvation, conversion and sanctification, our hearts or spirits are made and kept sin free and we can live in God’s kingdom of righteousness, a spiritual place that exists in our hearts, now in this life and forever.
When we human beings don’t know God, we live entirely in the natural realm by our senses and reason, knowing nothing about this sin free place in the spirit called the kingdom of God. This is where God’s presence is. But God in His wisdom, love and goodness to us has made it possible for everyone of us, without exception, to come to know Him, by giving us two irrefutable evidences of His existence, that even the natural human part of us can understand. The first of these is the evidence of nature itself, the wonder of His amazing creation all around us and in the skies above. The second is the evidence of an inner voice or conscience that instinctively knows the difference between right and wrong, a moral compass, that none of the rest of His creation has.
These two evidences are more than sufficient to motivate us to open the door of our hearts to allow God to reveal Himself, and for us to actually know Him, by believing the gospel. If, despite the evidence of creation and our conscience, we choose not to let God save us from our sins, we are then without excuse and will be judged for our sins.
Rom 1:20 From the time the world was created, people have seen the earth and sky and all that God made. They can clearly see his invisible qualities — his eternal power and divine nature. So they have no excuse whatsoever for not knowing God. NLT
Rom 2:12-15 God will punish the Gentiles when they sin, even though they never
had God's written law. And he will punish the Jews when they sin, for they do have the law...Even when Gentiles, who do not have God's written law, instinctively follow what the law says, they show that in their hearts they know right from wrong. They demonstrate that God's law is written within them, for their own consciences either accuse them or tell them they are doing what is right. NLT