In the ancient biblical world God Himself declared that there are somethings that are Holy or set apart from other things. In Exodus 16:23 God said: ‘Tomorrow (the seventh day of the week) is a Sabbath rest, a holy Sabbath to the LORD." In Exodus 20:8-11 God explained that while the Israelites had six days to work the seventh day was a day of rest where no work of any kind would be done. Later in Exodus 31:13 God would say "Surely My Sabbaths you shall keep, for it is a sign between Me and you throughout your generations, that you may know that I am the LORD who sanctifies you. "
Jesus kept this sign of sanctification in His life and declared that He was Lord of the Sabbath in Mark 2:28. This evidences that God intended that the Sabbath be eternally set apart from the rest of the week. At the very least there is one institution that reminds us that the dichotomy of sacred and secular continues to exist.
If God intended that the seventh day be set apart for remembrance and reverence does this imply that during the other six days we are allowed a Godless existence? Certainly not, but the distinction of sacred and secular still remains because of our human limitations. It is very important for us not to mix the sacred and the secular. At this point there are some that believe this type of thinking smacks of legalism sighting such well known texts as "in everything you do, do it all to the glory of God." But texts like these do not imply that there is not a distinction between the secular and the sacred.
Paul himself says in 1 Corinthians 46 "However, the spiritual is not first, but the natural, and afterward the spiritual." The physical comes first just as the six days of God's work came before the sacredness of the seventh day Sabbath and each week this cycle continues and will continue for all time.
There can be problems when people begin to artificially and independently make there own separations. For example when people believe they can lie and deceive outside the walls of the church but they need to be good and honest inside the walls they are missing the point. This is not the purpose of the distinction between sacred and secular. The purpose of this distinction is merely to provide a time and a place for human beings to the everyday things of life and a time and a place to allow the spiritual side to flourish. We are not without spirituality at the office, or at the construction site, but we are less able to focus on spiritual things.
The separation between what is sacred and secular is meant to optimize our ability to function in both places by providing a time and place for reflection that cannot exist to the same degree when we are concentrating on completing secular activities. This does not make the other six days of the week a time for debasement it only means that that there is a specialness in sacred things that allow us to pause and reflect on the greatness of our God.