What is faith?

In Philippians 3v8,9 Paul tells us that he considers everything rubbish compared to knowing Christ, and he has gladly swapped his past self-righteousness for the righteousness that comes through faith in Christ Jesus. But it's so easy for us to think that this 'faith' in Christ Jesus is somehow what we do, as if we contribute something to our salvation. Here's a great quote from Alec Motyer's BST commentary on Philippians:

“The great missionary John G. Paton, struggling to find a local word which would translate ‘faith’ and failing to find one, was interrupted by someone in great trouble and needing help. ‘Please may I come and lean heavily upon you?’ he said. Faith is leaning heavily upon Christ; not labour but cessation of labour, not doing but ceasing to do; simply leaning the whole weight of our needs upon Him, and finding in Him acceptance before the presence of God, and a righteousness which could never be ours by our own works.”