What does it mean / how do you know / how would you explain "God is working" or "God is at work"? Usually this phrase may come up during testimonies or used at church.

How would you explain it to someone who asks "How do you know it was God at work?" (usually an unbeliever)

Suppose someone has gone through major changes after accepting Christ. The saved person might say "God has been working" in their testimony. Another might ask "How do you know it was God at work? How do you know that person didn't make those changes on his/her own will?"

What passages might you include?

(If too subjective, leave a comment and the question can be deleted)
Early Christians were often exhorted to &quot;test every Spirit&quot; to see if it was of God. (1 John 4:1).
When the Bible was first received, there was very little question of whether or not spirits were at work, but rather which ones.  For this reason, the question &quot;Is it God&quot; isn't particularly within the domain of Scripture - or even doctrine.  God is assumed to be ultimately responsible for it all, so of course, it is &quot;of God.&quot;
As to the question of whether or not a person is saved, there is also a simple test.  As Matthew records Jesus' Sermon on the Mount, the key idea is &quot;by their fruits you will know them.&quot;

“Watch out for false prophets. They come to you in sheep’s clothing, but inwardly they are ferocious wolves. 16By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? 17Likewise every good tree bears good fruit, but a bad tree bears bad fruit. 18A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. 19Every tree that does not bear good fruit is cut down and thrown into the fire. 20Thus, by their fruit you will recognize them.
21“Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but only he who does the will of my Father who is in heaven. 22Many will say to me on that day, ‘Lord, Lord, did we not prophesy in your name, and in your name drive out demons and perform many miracles?’ 23Then I will tell them plainly, ‘I never knew you. Away from me, you evildoers!’