The history of bishops in the legislature is a long one:
At present, two Archbishops and 24 Bishops of the Church of England sit in the House of Lords.... The Archbishops of Canterbury and York receive a seat in the Lords as of right together with the diocesan Bishops of Durham, London and Winchester. The remainder are appointed on the basis of seniority from amongst the diocesan bishops.The Archbishop of Canterbury's staff at Lambeth Palace are meant to arrange for a duty bishop to attend the House on every sitting day, but it is not clear whether this system always works in practice.
....When bishops retire (at age 70) they do not retain their place in the Lords.... However, some former bishops and archbishops have been appointed as life peers....
....The established Church of Scotland is not represented (excluded after the Scottish Reformation in 1638), neither are the Church of Ireland (after it disestablished in 1871) and the Church in Wales (after it disestablished in 1920).
Historically, there had been a significant Christian element in the meetings of the Anglo-Saxon Witan. Bishops and Archbishops formed a sizeable part of the House of Lords since their inclusion in William the Conqueror’s Curia Regis. However, this number diminished heavily in the period of the Reformation and during the dissolution of the monasteries under Henry VIII. During the English Civil War, the Bishops were excluded from the House of Lords, the chamber being abolished during the period of the Commonwealth, but the Clergy Act 1661 made provision for their return. The number was limited to 26 (the number existing at the time of the legislation) by the Bishopric of Manchester Act 1846....
There is some statistical evidence on the bishops' involvement in the work of the House. Perhaps surprisingly, this seems to indicate a broadly increasing level of participation. In the 1980s, the average bishop was in attendance on 12% of sitting days, whereas evidence from 1997-2005 indicated a rising attendance rate, which moved above 18% by the end of the period. In the Thatcher years, bishops voted in 22.9% of divisions, and this figure had risen to 46.5% by 2001-05. Similarly the figures show that there was a rise in the number of bishops attending per day and the number of divisions in which two or more bishops voted.
This increasing participation has not, however, translated into a high level of influence, notwithstanding the recent debate on welfare reform. Meg Russell and Maria Sciara of the Constitution Unit at UCL have written:
[T]he votes of the bishops only rarely make a difference to legislative outcomes. Of our 806 divisions this occurred only four times.... Only once, on the Nationality, Immigration and Asylum Bill in 2003, was the government defeated thanks to the votes of bishops, when otherwise it would have won.The bishops aren't all that popular. While it doesn't come as a massive surprise that the British Humanist Association thinks that having religious leaders in the legislature is a bad idea, it must be significant that 70% of Christians don't think that they should be there either. As to the argument that they provide a valuable ethical or spiritual perspective on public issues, it is a little-known but striking fact that their interventions in debate are not usually theological in nature. The think-tank Theos has reported:
The Lords Spiritual made use of facts and statistics to support their points on 197 occasions and academic and professional advice on 149 occasions. Meanwhile, reference was made to theological arguments on just 77 occasions, and explicit reference was made to the Bible during just 7% of their speeches. Only 1% of the episcopal contributions to the House of Lords explored biblical material in detail.In the past, it has been suggested that other religious leaders should be admitted to the Lords to counterbalance or complement the bishops, perhaps together with prominent humanist and secularist figures (Lord Dawkins? Lord Grayling?). But this option isn't popular among the communities concerned. The Catholic Bishops’ Conference of England and Wales seems to be in favour of faith leaders being appointed to the second chamber, but there is a long-running debate over whether the Code of Canon Law even allows Catholic clergymen to sit in the Lords. The Network of Buddhist Organisations isn't sure how Buddhist representatives might be chosen, and the Board of Deputies of British Jews worries that the various different factions of British Judaism are divided enough already without trying to get them to agree on the appointment of parliamentary delegates.
All in all, there are strong arguments in favour of dispensing with the Lords Spiritual in the next round of Lords reform.