‘Faith’ in religion, and particularly in Christian religion, is a central word, and as with most central words in, say, a language more broadly speaking (such as ‘know’), contains multiple meanings and works in various directions.
For the word to start to make some sense to a secularist, however, it might be useful to start at this point: faith can refer not to belief that God exists, but rather to belief in a specific version of God’s character. This is a more natural way of using the word in everyday language – as in ‘I have faith that someone will show up at a certain time, because I have had repeated experiences in the past where they have done so’. The basic idea here is one that basically everyone acts on in day to day life – habit or character is inferred from behaviour, and so one has warranted belief that someone will probably act in such-and-such a way in the future.
This leads to the next question: how would one know that God has a character or something like it (is person-like) and discern what that character is?
There are 3 main areas of evidential support typically cited in Christianity, as far as one’s own experience goes. The first is providence – the idea that, usually in retrospect, one can see a pattern or logic to events in one’s life, even though at the time it might have seemed like there wasn’t. The typical Christian idea here is that God has an intention – a forethought or will – for how things will turn out (but that the actual turning out of things in that way depends on human choices). Through repeated experiences of these sorts of things, and developing a better ability to listen and communicate with God (through various practices) one can then better align oneself with and allow for God’s providence, and so build a sense of the sort of God there is.
The second is ‘coincidence’ – seemingly unlikely events where things come together in a certain way. These are distinguishable from providence in typically being noticeable at the moment. Christians attribute these sorts of coincidences to God, or angels which are proximate representatives of God’s will. Similar to providence, through repeated experiences and developing an ability to notice God’s feedback to one’s own thoughts through these sorts of coincidences, one starts to build an evidenced concept of what sort of character God has.
The third main source of evidence is ‘religious experience’ – experiences of the ‘light of Christ’ or the ‘Holy Spirit’, for example, or even just of a general sense of ‘goodness’ that is perceived to indicate the presence of some divine aspect.
Although the three sources of evidence discussed above have to do with the specific nature of God or divine reality, they also work as evidence that there is a God – another reason why the word ‘faith’ is often run together on these issues. Interestingly, in Kreeft and Tacelli’s Handbook of Christian Apologetics, of the 20 arguments for the existence of God, only the third source of evidence above is mentioned, in argument 17.
‘Faith’, then, in the context discussed above more properly refers to the character of a relationship – having faith in a specific notion of God, say, because of evidence of his character that has built up in the past.