The greatest testimony you can give to words the atheist is a changed life as it is clearly seen in Colossians 1:13 when we are delivered from the Dominion of Darkness and transferred into Kingdom of his light, delivered from all kinds of bad habits, delivered from all kinds of sin, and so forth, I'm not saying what you said is not true but I know a lot of atheists who are in severely bad accidents and they survived so called miraculously but I doubt very much that God was in it, newness of life is the greatest testimony But ultimately only the Holy Spirit can convince anyone of God's existence through John 16:8-9 the conviction of sin which is how I was born again