v 5. The purpose of my instruction is that all believers would be filled with love that comes from a pure heart, a clear conscience, and genuine faith.
Paul was writing to Timothy about the conduct of the Christian believers in Ephesus. (Paul experienced severe opposition, there including a civil riot, as you can read in Acts 19) The Amplified Bible translation adds, after pure in heart, those with integrity, moral courage, and godly character.
Those characteristics are certainly elements of a pure heart, and Paul adds here that love flows from a pure heart. Genuine love is an expression of a heart that is pure. This is love devoid of selfishness, guile, fear and duplicity. It is love that seeks the best for another person regardless of the cost to the lover. Paul is asking Timothy to teach the new Christians all about purity of heart and life so that love develops among them.
How is purity of heart to be developed? It begins in a person’s thinking; once the thought-life is brought under control then purity of heart will follow. Hence Paul’s admonition in Philippians 4:8 ‘And now, dear brothers and sisters, one final thing. Fix your thoughts on what is true, and honourable, and right, and pure, and lovely, and admirable. Think about things that are excellent and worthy of praise. ‘ Or again in Romans 12:2 ‘Don’t copy the behaviour and customs of this world, but let God transform you into a new person by changing the way you think. ‘ It is so easy to let thoughts wander – and not necessarily in a helpful and pure direction.
So the battle to be pure in heart begins in the thought life; challenge and correct your thought-life and purity of heart and action follow along after.