To be rational is to use our own experience of life, and the accumulated knowledge of others, to guide our decision making.
The four busy lanes I must cross to my office each morning can easily keep me waiting for five minutes or more. I watch the traffic. I gauge speed. I estimate distance. I allow a plentiful margin of error, calculate the effect of unexpected lane changes, and always, always watch the cars as I cross. Sometimes they aim at you and speed up!
We knew an old man who couldn't see well. Someone asked how he used to cross the road near his house; he couldn't see the cars. He used to stand on the kerb and listen carefully. When he could hear nothing, he would cross over! That too was rational- although risky! However, if I decide that at exactly 8.10a.m. I will cross the road- eyes shut, not listening- in the faith that nothing will hit me, that is irrational. It is to act completely inconsistently with all the evidence available to me.
Christianity is often practiced in deeply irrational way. For example, an engineer who is deeply logical in his work, denies rational biology and holds to seven day creation, and ad hoc creation-(pseudo)science. He would not consider such an approach in his own field; he cannot afford to have his bridges fall down. Yet he idolises an ancient book, holding it above criticism, so he may continue hold to a set of beliefs that are against all the evidence.
A key event of Christianity is the resurrection of Christ. Many hold to that in a literal way. We do not otherwise believe in people being raised from the dead. Once dead, a person is dead. And yet many Christians base their whole faith on the literal physical truth of that event. In the end, this is irrational. The depth of the irrationality is found in the fact that it is so hard for many Christians to see what is so obvious to those around them.
Ethically, Christianity has much to say which is relevant to life and valuable. The idea of compassion being pre-eminent over law is a key Christian concept. So too the radical call to justice for those who are poor. This ethic had a massive effect in the time of Lord Shaftsbury and others, and is still badly needed today. Indeed, as the rich get much richer in Australia, the need for radical discipleship is greater than ever. However no one will listen to the ethic of a faith that is irrational. Christianity needs to abandon its irrational basis and present a rational and reasonable hope if its ethic of good news for the poor and its ethic of mercy is to be heard by the society around it.