Holiness means to be set apart by God. When we talk of God desiring holiness from His people we mean that He is calling us to be free from sin and the world and to live a life totally dedicated to Him.
When we become a Christian our sins are washed away by the blood of Jesus and we are made righteous and holy in His sight “God made him who had no sin to be sin for us, so that in him we might become the righteousness of God.” (2 Corin 5v21)
As soon as we put our trust in Jesus, we are given the gift of eternal life and we are made holy before God (Rom 10v13).
However, once we are saved we must continue to walk in holiness. Jesus tells us that it is the pure in heart who will see God (Matt 5). John tells us “Do not love the world or anything in the world” (1 John 2v15). He also tells us that we must “walk in the light” (1 John 1). God demands that we live holy lives because He Himself is a Holy God “Be holy, because I am holy” (1 Peter 1v16). He has called us to be a “holy nation” (1 Peter 2v9)
The consequences of us not living holy lives before God are serious. Although we are not saved by our works, Jesus Himself said that the fruit of our lives was evidence of whether we truly belonged to Him “A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit.” (Matt 7v18). Paul wrote and warned the Corinthians about their sinful practices. (1 Corin and 2 Corin). He also wrote that we must all stand before the Judgment Seat of Christ (2 Corin 5v10) and give an account of our lives.
Until we get to heaven none of us will be completely without sin and we will battle to live holy lives. At times we will sin and fall short of Gods standard. However, we are to desire every day to become more like Jesus and be changed from glory unto glory.
In order to become more like Jesus we are count ourselves dead to sin (Rom 6v11) and have our minds renewed by the word of God (Rom 12v2). We are not to live as the world lives but instead are to walk in obedience to the Word of God.
God has given us the Holy Spirit to live inside of us. It is He who will sanctify us and change us as we yield to Him (Rom 15v16). It is He who will give us the power to overcome sin as we give our lives totally over to God.