There are two equal and opposite heresies being enacted by Christians in the UK, and they are most visible when Christians engage in politics. They are particularly important because they relate to the very nature of God.

The first heresy is that God isn't really real. God is essentially an imaginary friend to whom we speak occasionally when life gets tough but otherwise has little or no influence on our daily lives. Though this view may not be articulated clearly by the people who hold it, actions speak louder than words. This virtual atheism means that in the big decisions of life, God is absent. He doesn't shape our career decisions or consumer choices and he most certainly doesn't do politics.

Christians who practise this heretical form of Christianity engage in politics along the same lines as everyone else – by looking at the policies that affect us most and vote according to our personal interests and agendas. This is why tax is often a major part of an election campaign; politicians ask us to do the maths and try to convince us that we will be personally better off under their governance.

The second heresy is that God belongs to Christians; that he isn't really Lord of all creation but is instead some kind of tribal deity. God is simply there to help Christians enjoy life more. At a personal level it might mean speaking to God as if he is a personal concierge, therapist or masseuse. When we're not ordering God about we are offloading our emotions onto him or telling him which parts of our bodies hurt. Somehow we have reversed the roles we were given: we are the kings and God is our servant.

At a corporate level, we expect God to support the Church. He is our God, he is for us and so no one can stand against us. This heresy means that we engage in politics to get a Christian agenda through. We write to our MP when we want them to adhere to Christian values on the hot-button topics such as abortion, euthanasia and gay marriage. We expect the government to comply on these issues because they are the kinds of issues that we believe God is interested in.

Forgive the caricaturing. Sadly, I have spoken to a number of politicians who have told me about their experience of engaging with Christians and it hasn't been great.

So I was excited to see a new initiative from old friends of mine at Christians in Politics and the Evangelical Alliance. It's called the 'Show Up' campaign, which launches today. It's calling Christians to engage in politics – not just by voting in the general election in May but by getting stuck into political life. The campaign's title comes from a phrase popularised by the hit TV show West Wing: "Decisions are made by those who show up."

Showing up to vote is the least we can do, not just because so many people around the world can't influence their country's governance and wish they had the opportunity to vote, but because as citizens of the United Kingdom and of heaven we are called to live out the values and priorities of God's coming kingdom now. This means that in our political engagement we demonstrate that God is real not imaginary, he is interested in our public life not just our private feelings and he has love and compassion for the whole world, not just for Christians.

'Show Up' brings together Christians from all the major parties. As I have had the opportunity to get to know a number of Christian politicians from across the political spectrum, I have been really encouraged by their shared vision for demonstrating the grace, compassion and justice of God in their political lives. I have also been impressed by the sense of mutual respect and honour that Christian politicians from all the major parties show each other despite their considerable political differences.

I recently heard a story of Christians 'showing up' in Southampton. Church leaders went to see their local council when they heard that the austerity measures were coming and that local service provision was going to be cut. They decided not to start a Twitter campaign against the council or the government, they decided against just holding a prayer meeting for the needs of the city. Instead they went to see leaders from their council and asked how the church could bless the city.

I can imagine the council leaders' shock as Christians were coming not to wag a finger at them but to offer a helping hand. The church leaders were told that the council could use the churches' help in two areas: working with young people and in finding 40 more foster carers to meet the city's growing number of children at risk, both of which the churches were happy to do.

So I want to encourage you to show up in the public life in your town. Let your life challenge the heresies of an imaginary God who is only interested in our private feelings. As opportunities come up to influence the community where you live, do it in a way that demonstrates God's love for all people, his passion for justice and his concern for the vulnerable. As the Church let our engagement in the political life of our town last longer than the elections, be more than gathering for hustings and asking the politicians where they stand on hot-button issues.

Perhaps showing this video in your church or home group will help to catalyze the change needed in our political engagement and make sure we demonstrate through our words and actions our belief in a real God who really does love the whole world.

Dr Krish Kandiah is president of London School of Theology and founder and director of Home for Good, a brand new charity helping to find loving homes for every child in care that needs one.