The relationship of faith and reason has often been controversial. As early as the third century the Church father Tertullian asked, 'What has Athens to do with Jerusalem, the academy with the church?' Many Christians today - even professional Christians and Christian students - have a view of faith that is essentially irrational. Their professional life or studies are in one compartment, their faith in another. This anti-intellectual stance has important practical consequences, because unthinking Christians often do things that are mindless or whacky, hold attitudes that put others off from becoming Christians, are ill-equipped to explain Christian faith to interested enquirers, and abandon intellectual and public life to secularists.

In actual fact, there are strong Biblical grounds for affirming reason and rationality:

a.. Jesus told us to love God with all our mind as well as all our heart (Matthew 22:37, thus endorsing the Shema, the Jewish confession of faith, Deuteronomy 6:5).

b.. Paul said that the offering our bodies to God was not an irrational or foolish act but our 'reasonable worship' (logikos, a logical act in accordance with true goal of our lives), to be accomplished by the renewing of our minds (Romans 12:1).

c.. Peter urged us always to be ready to answer unbelievers or enquirers who ask us 'to give the reason' for the hope we have and the faith we hold (1 Peter 3:15).

d.. The supreme biblical affirmation of the place of reason is the Prologue of John's Gospel, where the Stoic or neo-Platonist term logos ('word', 'reason') is applied to the pre-existent Son of God through whom everything was created in the beginning and who enlightens every human being born into the world (John 1:1-4, 9). Here the Logos is the principle which unifies all reality and renders that reality intelligible to us.

Christians can use reason in two ways. Firstly to provide evidence for the truth of the Christian faith. Secondly to show shortcomings or inconsistencies in the views of non-believers. The former approach is positive or constructive apologetics. The latter is negative or critical apologetics. The first shows the reasonableness of Christian faith, the second shows the foolishness of unbelief.

from http://www.stalbans.org.nz/teachings/rob_yule/apologetics/apolog1.htm