"Speak unto all the congregation of the children of Israel, and say unto them, Ye shall be holy: for I the LORD your God am holy." Leviticus 19:2
"Ye shall be holy: for I the Lord your God am holy", was not meant for the priest only but for the whole congregation of Israel. God ordered Moses to deliver the summary of the laws to all the congregation of the children of Israel; not to Aaron and his sons only, but to all the people so that they can know their duty and what the Lord required of them. Moses must make known God's statutes to all the congregation, and proclaim them through the camp.
Today, the message of 'be ye holy', is to everyone who will like to follow Christ. Walking with Christ requires holiness of life because two can't walk together except they be agreed, Amos 3:3. Believers are called to relinquish the former lust of ignorance, unholy, excuse giving, indulgent living and be holy to walk with Christ because Christ is holy. 1 Peter 1:14-16
GOD'S INSTRUCTION TO HIS CHILDREN
God required that Israel be a holy people, because the God of Israel is a holy God. Holiness is to distinguish them from the other people to teach them a real separation from the world. The separation is not to physically isolate them from other nations but to distinguish them from the ungodly customs and practices of those nations so as to live to meet God's standard in entire devoutness to God as in 2 Corinthians 6:17,18. 1 Pet 1:15, 16. We are the followers of the holy Jesus, and therefore must be, according to our capacity, consecrated to God's honour, and conformed to his nature and will. Israel was sanctified by the types and shadows, but we are sanctified by the truth, or substance of all those shadows, John 17:17; Tit 2:14.
GENERAL INSUFFICIENCY IN HUMAN CHARACTER
God's instruction through Moses was violated because "the spirit indeed is willing, but the flesh is weak." Matthew 26:41. Israelites under pressure often forget due to human frailty, sometimes temptations presses them so much that in despair they cross the line succumbing to the flesh making them insufficient of God's holiness. God decided in Hebrews 8:10 that he will write the law in the heart to help His people not to violate or forget His laws. Isaiah categorized our human efforts as "filthy rags", Isaiah 64:6, insufficient of God's holiness and righteousness no matter how we try.
GRACE INDUCED THROUGH HEAVENLY CONNECT
Isaiah's experience gives an idea of how to be holy as God is holy in Isaiah 6:1-7. The vision of God gave Isaiah the idea of the holiness of God, beyond human effort, religious obligations, piety, self righteousness etc.
Isaiah saw God upon his throne, John 12:41 explained that he saw the Lord Jesus, saw Christ's glory and spoke of him. He saw his temple, his church on earth, filled with the manifestations of his glory, saw the bright and blessed attendants on his throne, in and by whom his glory is celebrated and his government served.
He saw the angels, the seraphim, have heat proportionable to their light, have abundance, not only of divine knowledge, but of holy love. He noticed their wings, they had each of them six wings, not stretched upwards (as those whom Ezekiel saw, ch. 1 11), but, 1. Four were made use of for a covering, as the wings of a fowl, sitting, are; with the two upper wings, next to the head, they covered their faces, and with the two lowest wings they covered their feet, or lower parts. This bespeaks their great humility and reverence in their attendance upon God, for he is greatly feared in the assembly of those saints, Ps 89 7. They not only cover their feet, those members of the body which are less honourable (1 Cor 12 23), but even their faces. Though angel's faces, doubtless, are much fairer than those of the children of men (Acts 6 15), yet in the presence of God, they cover them, because they cannot bear the dazzling lustre of the divine glory, and because, being conscious of an infinite distance from the divine perfection, they are ashamed to show their faces before the holy God, who charges even his angels with folly if they should offer to vie with him, Job 4 18.
Isaiah connection to God brought a real experience. To be holy, we must allow heaven to do it not just by trying to be holy but by God working in us "For it is God which worketh in you both to will and to do of his good pleasure." Phillipians 2:13.
The divine nature will change our human character to heavenly, godly character. When the flesh pushes us to evil the divine character will remind us we are no longer living in the flesh, when the temptation comes the divine character will initiate the courage to say 'no' in us and give us the victory. That is the godliness we acquire through grace and it is called holiness.