For since the creation of the world, God's invisible qualities - his eternal power and divine nature - have been clearly seen, being understood by what has been made, so that men are without excuse. - Romans 1:20
We have all seen God's invisible qualities. This phenomenon is known as General Revelation. As the name suggests, it is a revelation to all of mankind that there is a divine Creator of the universe.
In a broad sense, all of creation bears certain qualities of God. The universe, itself, is vast and seemingly endless. The oceans are powerful and unconquerable. Living things are diverse but unified in so many ways.
But there is one form of life that more specifically exemplifies the phenomenon of general revelation: mankind. The Bible tells us that humans were created in the image of God. Nothing in creation comes closer to displaying the fullness of God's nature than human beings. We have the ability to reason, create, love, and tame the world around us. When we look upon each other, it ought to be obvious that we were created with something special in mind.
Unfortunately, because of the curse of sin, nothing in the universe more closely resembles Satan than mankind. Jesus not only called his religious adversaries children of Satan, but called his disciple Peter Satan as well! Another disciple of Jesus, Judas Iscariot, was indwelt by Satan. Judas, as a result, became so filled with hate that he betrayed the Son of God into the hands of those who wanted to kill Him. Mankind is directly responsible for the greatest evils the world has known.
On the other hand, Jesus made a great promise to His disciples before He was crucified: "Before long, the world will not see me anymore, but you will see me. Because I live, you also will live" (John 14:19). Jesus is God in the flesh. We are born in the image of God, but God lives inside of those who believe in Jesus.
When you believe in Jesus and are in a community of believers you will see Jesus in your midst. He lives inside of you and the Church is the body of Jesus.
The world has absolutely no excuse to reject God. It has the general revelation of nature, the general revelation of mankind, the Special Revelation of Jesus (the God-Man), and God the Holy Spirit living within the community of His Church.
God might be invisible, but He can be clearly understood by what He has made. His eternal power and divine nature are on display through those who love Him and serve Him. Today, if you believe, you can see Him too.