At the time that Measure for Measure was written, England was a Protestant country, meaning that the monarch at the time, James I was the head of the Church of England. However, the England that Shakespeare was brought up in was still living with the remnants of a Catholic history, and so the religious beliefs that he would have learnt about as a child would have been those of the Catholic faith. Although Shakespeare was writing the play for a Protestant audience and a Protestant king, he was setting the play in a Catholic country, so this gave him an opportunity to feature his religious beliefs as a Catholic in the play.
One of the characters in the play particularly affected by religion is the Duke, as he discards his secular identity and is able to take on a religious identity as a friar, which allows him to behave in a particular way which was not possible for a Duke. The fact that the Duke is disguised, and really has another identity makes it easy for the audience to see how religion affects the way he behaves, and also the way other people react to him, as we can compare him as a religious figure, to him as the ruler of Vienna. One aspect of the Duke as a friar that can be compared to him as a ruler is the amount of power that he has. The start of the play shows the audience the Vienna for which the Duke was responsible, and the audience are shown him taking a break from Vienna and leaving someone else to restore law and order. The audience learn that people are allowed to get away with breaking laws regarding fornication, and that people like Mistress Overdone run brothels without a problem. Although the Duke is supposed to have power, he doesn't choose to use it, and forgives people who break the law and doesn't punish them. However, when he gains status as a religious figure when disguised as the friar, he is able to influence people's decisions, for example, persuading Isabella that asking Mariana to sleep with Angelo is a good idea, and then persuading Mariana. It is because of the character's religious beliefs that they believe him and trust his judgment, and will do what he tells them.
When the Duke is disguised as a friar, he seems to represent the Bible, especially the New Testament, as he is promoting forgiveness as opposed to the Old Testament, which seems to be more represented by Angelo, and his ideas about "An eye for and eye and a tooth for a tooth." When the Duke speaks to Claudio, the ideas he offers on death are very similar to those shown in the Bible. For example, he suggests that death is like sleep, and this makes a parallel with the Bible, where the dead are referred to as "Those who have fallen asleep" Although this would not be obvious to a modern audience, the audience at the time of Shakespeare would have understood references to the Bible, as religion was a much larger part of their society than of ours.
The Duke is also shown as a figure representing Jesus, as he is the person who...