Holiness is a word that refers to what humans experience as sacred, special and unique about God, that makes God worthy of awe and reverence, which above all calls for worship and obedience from those who become aware of it. Isaiah's vision of God's presence in the temple (Is 6:1-8) well describes this experience. Remarkably the Jewish Torah makes it clear that human beings are regarded by God has capable of possessing the same qualities. "Be holy, for I, the Lord your God, am holy." (Lev 19:2) It follows from God's declaration about human nature in the first Genesis creation story "Let us make mankind in our image, in our likeness." The potential for holiness is in each person, to be realised.
The notion of holiness described by an experience offers no guidance about how to become a holy person, however. The Jewish Torah is very clear about this. Holiness belongs to those who live genuinely by God's Law and Commandments. What this requires is obedience to the values of morality and justice embodied in the laws that give the Torah its name. These were summed up by Jesus in the saying that captures the essence of a huge code of social and religious regulations; 'You shall love God ... and your neighbour as yourself.' (Mark 12:30-31) Paul speaks about love being 'the fulfilment of the law'. (Rom 13:10)
In Matthew's Gospel, Jesus uses the image of separating sheep from goats in his parable about the last judgement, when all people must account for their lives. (Matt 25:31-46). He doesn't speak about the Law or laws as such, but gives concrete practical instances of what loving God and neighbour means, and what the consequences of action or inaction will be. It gives an everyday example of love in action, and from that can be gained an idea of how to recognise true holiness.
"Anything you did for the least of my brothers you did for me" says the Lord in the parable.
Loving service of others can thus be an expression of worship and devotion to God. It is the good example which Jesus set in the way he lived from start to finish, of obedience to God's law in letter and spirit i.e. in practical essentials and right intentions. He distinguishes between trivial elaborations generated by false piety, and core moral and spiritual values.
"For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished." (Matt 5:18)
Matthew's Gospel especially presents Jesus as the New Moses, who fulfils the law by his obedience even to death on the cross. His sacrifice reveals the true difference between the trivial and the essential in what God requires of his children.
'I long for your salvation Lord, and your law gives me delight.
Let me live that I may praise you, and may your laws sustain me.
I have strayed like a lost sheep, Seek your servant,
for I have not forgotten your commandments.' (Ps 119:174-176)
'Lord, may my ways be steadfast, in doing your will' (Ps 119:5)