God called the Israelites to holiness when they were in the wilderness, long before they crossed the Jordan River:
Sanctify yourselves therefore, and be ye holy: for I am the LORD your God (Leviticus 20:7).
That He wants us to be holy because He is holy, cannot be a surprise considering the Bible tells us we are made in His image:
And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth. So God created man in his own image, in the image of God created he him; male and female created he them (Genesis 1:26-27).
Strong's definition of holy in Hebrew is:
"qôdesh, ko'-desh; H6944 from H6942); a sacred place or thing; rarely abstract, sanctity:—consecrated (thing), dedicated (thing), hallowed (thing), holiness, (most) holy (day, portion, thing), saint, sanctuary."
From this definition we can key on the word "sanctity (or sanctify)", which comes from the root word of holy H6942, and helps us to understand what God expects from His children, the most high saints (Daniel 7:18). In the Bible, H6942 is used to mean "to consecrate, sanctify, prepare, dedicate, be hallowed, be holy, be sanctified, and be separate". We can immediately see why God separated the Israelites from the rest of the world. They were to be different than the rest of the world:
And ye shall be holy unto me: for I the LORD am holy, and have severed you from other people, that ye should be mine (Leviticus 20:26).
In this verse, we see that the word used here to distinguish His people from the world is "severed", that is, completely cut out from the world. He wants the same thing for us today.
If God wanted His chosen people to be holy in the Old Testament, would He expect anything different from Christians today? Of course not! God's intentions all along were to allow the Gentiles to be grafted into the same tree as the Israelites as the Apostle Paul explains in Romans 11:1-36. When we read this chapter, we see that this grafting is conditional. He explains that the Jews were enemies of the gospel of Jesus Christ. This fact opened the doors for the Gentiles to replace the Jews because of their hard hearts and will continue until the "fullness of the Gentiles", which means the second coming of Jesus. Thus the condition is that we cannot be enemies of the Gospel. If that is the case then we must love and obey the gospel of Jesus Christ. We must be born again!
Obviously then, if the Israelites were called to be holy, so much more His born again children.
To the end he may stablish your hearts unblameable in holiness before God, even our Father, at the coming of our Lord Jesus Christ with all his saints (1 Thessalonians 3:13).
Jesus prayed the following prayer for His disciples after the Last Supper:
I have given them Your word; and the world has hated them because they are not of the world, just as I am not of the world. I do not pray that You should take them out of the world, but that You should keep them from the evil one. They are not of the world, just as I am not of the world. Sanctify them by Your truth. Your word is truth. As You sent Me into the world, I also have sent them into the world. And for their sakes I sanctify Myself, that they also may be sanctified by the truth. (John 17:14-19 NKJV).
We clearly see that just like Jesus, His disciples were "not of the world". He did not want God to take them out of the world, but instead protect them from the evil one, the devil. Because they were sanctified by the truth of God, He was sending them into the world to spread the gospel (Mark 16:15-17). We know that is so because that is exactly what they did the day of Pentecost, after Jesus' ascension. The Apostle Peter preached the first gospel message in Acts 2:14-39.
I say all that to remind us as born again Christians, that we are Jesus' disciples and have been chosen us to be sanctified, like Him, like His disciples, and to spread the gospel so other Gentiles would be sanctified as well. The Apostle Paul said it this way:
That I should be the minister of Jesus Christ to the Gentiles, ministering the gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Ghost (Romans 15:16).
Getting to the heart of the matter, as the temple of the Living God, we must be free of sin, cleansed from all filthiness of the body and spirit, separated from those that commune with unrighteousness, producing fruit unto holiness and perfecting it out of reverence for God, becoming His sons and daughters, allowing us to spend eternity with Jesus Christ:
But now being made free from sin, and become servants to God, ye have your fruit unto holiness, and the end everlasting life (Romans 6:22).
Be ye not unequally yoked together with unbelievers: for what fellowship hath righteousness with unrighteousness? and what communion hath light with darkness? And what concord hath Christ with Belial? or what part hath he that believeth with an infidel? And what agreement hath the temple of God with idols? for ye are the temple of the living God; as God hath said, I will dwell in them, and walk in them; and I will be their God, and they shall be my people (2 Corinthians 6:14,15,16).
Wherefore come out from among them, and be ye separate, saith the Lord, and touch not the unclean thing; and I will receive you. And will be a Father unto you, and ye shall be my sons and daughters, saith the Lord Almighty (2 Corinthians 6:17,18). Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God (2 Corinthians 7:1).
We must always remember that if we have unrepentant sin in our lives, our past holiness will not get us to heaven:
Follow peace with all men, and holiness, without which no man shall see the Lord (Hebrews 12:14).
We must live a repentant lifestyle!