Psalm 15:1: “Lord, who may dwell in your sacred tent? Who may live on your holy mountain?”
God is a holy God. He has never sinned, nor tempted anyone else to sin. Anything He thinks, says or does is holy. Heaven is a holy place, because God lives there. Now, I think I would be right assuming that anyone who expects to go to heaven after death must be holy too. If you continue reading 15th Psalm you will see that for yourself. It deals with holiness and godly life style.
Let us also not forget that it should be much easier for us to lead that kind of life nowadays that it was for those people who lived back then in David times. Jesus has already died for us. He sent us his Holy Spirit to help us to fulfill the will of God and to live the way God likes. I do not mean we will attain absolute holiness here on earth and sin no more. No, but I mean we should pursue holiness. That should be our target and goal of a lifetime. And if we sin, He is righteous and merciful to forgive us any sin and cleanse us from all unrighteousness.