What is the beauty of entire sanctification?
It is being delivered from the power of sin; loving God with all the heart, soul, mind, and strength; and loving one's neighbor as one's self.
Catechism for the Global Methodist Church, question 71
18You shall not avenge, and you shall not keep a grudge against the sons of your people, but you shall love your neighbor as yourself. I am the LORD.
5And you shall love the LORD your God with all your heart and with all your soul and with all your strength.
30And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength. 31The second is this: You shall love your neighbor as yourself. There is no other commandment greater than these.
9For the commandments, You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet, and if there is any other commandment, are summed up in this word: You shall love your neighbor as yourself. 10Love does not work evil to the neighbor; therefore love is the fulfillment of the law.
13For you were called to freedom, brothers; only do not use your freedom as an opportunity for the flesh, but through love serve one another. 14For the whole law is fulfilled in one word: 'You shall love your neighbor as yourself.'
23May the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ.
Having said that entire sanctification is possible, the catechism now says what it is. The answer is taken from Article XI of the Confession of Faith, which describes the state of perfect love as obtained "by being delivered from the power of sin, by loving God with all the heart, soul, mind and strength, and by loving one's neighbor as one's self." The three phrases come straight from the article, and the two commandments come straight from Jesus. Wesley defined Christian perfection the same way.
Delivered from the power of sin comes first, because love cannot fill a heart still ruled by something else. Article XI describes it as a cleansing of the heart by the Holy Spirit, received by faith. Paul's prayer for the Thessalonians asks for exactly this: "May the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ" (1 Thessalonians 5:23). "Completely" and "whole" are the words behind the adjective "entire."
Loving God with all the heart, soul, mind, and strength is the Shema of Deuteronomy 6:5, which Jesus names as the first commandment: "you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength" (Mark 12:30). Loving one's neighbor as one's self is Leviticus 19:18, which Jesus names as the second, adding, "There is no other commandment greater than these" (Mark 12:31). Paul then shows how far the second reaches. Every commandment about the neighbor is "summed up in this word: You shall love your neighbor as yourself," so that "love is the fulfillment of the law" (Romans 13:9-10). To the Galatians he says the same, "For the whole law is fulfilled in one word" (Galatians 5:14), and warns that Christian freedom exists "through love" to "serve one another" (Galatians 5:13).
The question calls this a beauty. Entire sanctification is a life in which the two great commandments have become the actual shape of a person's desires. For a Methodist the goal of the Christian life can therefore be stated plainly: to love God entirely and to love the people God has placed nearby as one loves oneself, and to be set free from whatever in the heart resists that love. Article XI says this state is one "every regenerate believer may obtain."