What Is Your Best Answer When Asked to Prove God Exists?

Questions about our best answer when an atheist asks us to prove God exists, how God can be merciful and forgiving without the existence of fallen humanity, and why, if God is complete in himself and already receives glory from the angels, he would create us to witness his glory.   What is your best answer when an atheist asks you to “prove God exists”? Since God is a Trinity, he can be love without needing creation, but how can he be, by nature, merciful and forgiving without fallen humanity, and wouldn’t that make us necessary? If God is complete in himself, he didn’t need to create us, and if his angels are there, he already receives glory, so why is it to the benefit of an uncreated creation that they should be created and witness the glory of God?