God is all about conforming us to Christlikeness and we can know exactly what that looks like by observing Christ in the gospels and reading his teachings in the entire NT. Perhaps the most descriptive passages of what holiness looks like are found in the Beattitudes in Matt. 5, such as being poor in spirit and faithfully enduring hostility for the sake of Christ, and then also the fruit of the Holy Spirit in Gal. 5, which includes kindness, patience and self-control. These characteristics represent expressions flowing from one’s heart, which is where true holiness must reside. While some can mimic these things for a short time, no one can truly attain to what is written there apart from divine power.
On the contrary, if we adopt our own ways of being holy, we deviate from God’s word and provide a stumbling block to gospel ministry. Holiness is defined by all of what Christ has said, so if we overemphasize certain teachings at the expense of others, we are not living holy lives. Most commonly, it happens like this—We develop certain out-workings of biblical teachings (such as do not love the world) and then we make those out-workings equal with God’s authoritative revelation and self-righteously judge those who don’t engage in the same practices. We can do this with specifics on media intake or even some of the spiritual disciplines, such as bible reading or prayer. This is what the Jews had done with God’s revelation in the books of Moses, which led to their contempt towards Gentiles. So in order for the gospel to go forth to the Gentiles from the Jewish apostles, these barriers would have to be permanently broken. Acts 10 is the account of how God accomplished that.