People who are agnostics are often dismayed with the sufferings in the world and cannot reconcile them with the idea of a good God.

The word 'agnostic' means literally in Greek, 'not knowing'.

As with anyone, pray for them, but the way to talk with them is TO SHARE THE GOOD THINGS YOU KNOW GOD HAS DONE.

It may take time, but give them the evidence of testimonies, such as you can find on this website or other sites, which point to the reality of the goodness and power of God.