A recent survey suggested that the 'good deeds' of American Christians go mostly unnoticed by the average US citizen. In some ways, it was good news perhaps that believers weren't pompously parading their charity to the world so that everyone knows about it. Then again, the negative consequence was also raised – if people don't know they can get help from the Church, then they won't ask for it.

It probes a paradox at the heart of Christian living: are good deeds meant to be public – a living light to the world – or purely private, as hidden and humble as can be? Jesus seems to have said both, but is that really possible? Did Christ contradict himself?

Jesus seems to have been unrelenting in his judgment on those who advertise their piety to the world. In the Sermon on the Mount, he warns: 'Be careful not to practise your righteousness in front of others to be seen by them. If you do, you will have no reward from your Father in heaven.

'So when you give to the needy, do not announce it with trumpets, as the hypocrites do in the synagogues and on the streets, to be honoured by others. Truly I tell you, they have received their reward in full.But when you give to the needy, do not let your left hand know what your right hand is doing, so that your giving may be in secret. Then your Father, who sees what is done in secret, will reward you' (Matthew 6:1-4).

They're challenging words, and Jesus gave similar warnings about prayer and fasting – they should be done in secret. We all know that the feeling of 'doing good' can be alluring, such that we focus on that more than the purpose of these deeds – actually helping people in need. The danger is even greater when, as in our present culture, social justice is actually pretty fashionable. Causes are cool. In a twisted way, we can use charity to make us look good and feel great – but the needy get forgotten.

So, Jesus says, don't do it. Then again, he also said the opposite. Only the chapter before, in the very same sermon, Jesus implores his audience: 'You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.

'You are the light of the world. A town built on a hill cannot be hidden.Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house.In the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven' (Matthew 5:13-16).

Here, we're told to be bright and bold, to let our deeds be so public that they shine before the world. And again the logic holds up: the Church is actually meant to do things on earth, to illuminate, heal and preserve what's good in creation. To defend the helpless and speak for the silenced. It can't do that if it's hidden.

Likewise, when Jesus tells people to 'turn the other' cheek when struck, or go two miles when you're only forced to go one, these are pretty public acts. They send a message. As the biblical scholar Tom Wright would put it, Jesus declared that God was becoming King on the Earth, taking charge, establishing a radical, counter-cultural alternative reality that took shape through the people of God. It has to be public.

So which is it – public or private? Did Jesus deliberately demand the impossible?

What Jesus seems to be doing is going beyond external acts to the human heart. The problem of the hypocritical Pharisee looks external but is really within: he wants the prestige that comes from public piety – his lengthy prayers betray his true audience. He's not truly seeking God. To combat that spirit, Jesus says – go into your room and pray, where no one will see you but your heavenly Father. Likewise, don't use your giving to make you look good – give in private, where you'll get nothing in return but the gift of giving itself.

In the opposite direction, Christians are called to a heart that looks outward to the world: humble in spirit, but also abounding in generosity. Our lives exist for the sake of others, and so we're implored to be a light that shows other the way; to stand up and show a weary world a hint of a new reality. In this case, when there are people in need, the life of a purely private faith is too constrained –  it can't be seen, and so people can't be helped.

The Christian faith abounds in paradox, and this ethical conundrum is another. But perhaps the complexity of the human heart demands something a little bigger than we can comprehend. We're pulled in directions that appear to contradict, because if we only had one imperative (eg be as public as possible, all the time) we might start thinking we had faith and God in a box. Instead, we're called to be inward and outward, humble and bold, to think much of the world, and not too much of ourselves – to know with godly wisdom when to stand out and when to step back.

Jesus famously preached on that mountain: 'Be perfect, therefore, as your heavenly Father is perfect' (Matthew 5:48).

It probably wasn't meant to be easy.

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