I was 22 and had believed myself a Christian my whole life, but now I wasn't sure. I was studying for an MA in theology at Regent College in Vancouver. We had a visiting speaker that day and he wasn't holding back. 'Faith without works is dead,' he roared. 'God's heart is for the marginalised, the poor, the oppressed. True worship is meeting the needs of the world.'

Needless to say, he had a very impressive story of his own to share – giving up a comfortable, half-hearted life in Canada to live four days' donkey ride into a poverty-stricken Latin American country as far as I recall, returning to set up a cutting edge social justice charity. Shame caused my face to burn and a feeling of hopelessness settled in the pit of my stomach. I was studying full time and had three part-time jobs. I had no capacity for more commitments but oh, how I wanted to be a Christian, and the longer he spoke, the more I doubted my right to claim that label.

The needs of our world are endless, the suffering unending. The challenge to respond comes from passionate ambassadors for specific causes, from Facebook posts, from news bulletins and personal encounters. Drug addicts. Refugees. Orphans. The homeless. Prisoners. The mentally ill. Give your money, your time, your expertise, your spare room, give until you bleed, spend yourself until you are used up and even then keep giving. Otherwise your declarations of faith are empty words.

One of the most frightening things Jesus ever said was 'Not everyone who says to me, "Lord, Lord," will enter the kingdom of heaven, but only the one who does the will of my Father who is in heaven' (Matthew 7:21).

I stumbled out of the lecture hall, tears blurring my vision, and walked straight into another professor. This minor kerfuffle was my saving grace. I left our short encounter deeply reassured. This is the essence of what he said, mixed in with some of what I have gleaned over the subsequent years:

God convicts, he doesn't condemn

Discipleship is worked out in gritty, costly obedience and there is no getting around that. However, love comes first – God's love towards us, primarily. Psalm 103 says, 'He does not treat us as our sins deserve or repay us according to our iniquities. For as high as the heavens are above the earth, so great is his love for those who fear him' (10, 11) .

We are not supposed to do everything ourselves

The English language has a major limitation: there is no plural 'you'. This means we read every imperative as an individual challenge, but clearly no individual can respond to every need. We are together the body of Christ and together we can cover all the bases.

For everything there is a season

As a student, my priority was to study. As a mother of tiny children, my priority was to care for them. If we are not careful, in an effort to tick all the boxes at once we take on too much and do everything badly (at best) or burn out (at worst).

We are never going to be able to earn our way into God's favour by good works – the glorious truth is that his favour is there for the least deserving as pure gift: 'For it is by grace you have been saved, through faith – and this is not from yourselves, it is the gift of God— not by works, so that no one can boast' (Ephesians 2:8-9). The more deeply we understand that, the more joyfully we will find our part to play in responding to the needs of our world.

Jo Swinney is an author, speaker and editor of Preach Magazine. Her latest book, Home: the quest to belong (Hodder & Stoughton) is out now. www.joswinney.com @joswinney