The crucible for silver and the furnace for gold,
    but the Lord tests the heart. - Proverbs 17:3

The idea of testing here is not about grading the heart; it's about refining the heart. 

When gold is put through fire, impurities rise to the surface so they can be scraped off, and then the gold is purified. Likewise, the Lord puts the heart through a kind of testing that brings impurtiies to the surface so it too can be refined. Strengthened. Made more beautiful. 

How does God do this testing? 

His word revealing truth to us. 

The world's brokennes revealing our false gods. 

Sinful people around us revealing our lack of love. 

Blessings revealing our pride and forgetfulness. 

In other words - everything. God will use anything and everything to reveal new areas of growth and then walk us through seasons of becoming more like Jesus. This promise for God's children is echoed later in Romans 8:28-29.

In what way is the Lord purifiying my heart right now? That is my prayer today. 

In what ways is he purifying your heart today? 

Are you submitting to it with joy and expectation? Or grumbling and foot-stomping? Give that some prayer today.