Paul wrote in Romans 8:33, “Who shall bring a charge against God’s elect? It is God who justifies.”
There is a big, powerful answer to Paul’s question at the beginning of this verse: how about Satan, whose very name means “the accuser”? And if it’s not Satan who is bringing charges of sin and guilt against me, then it is the world all around me and in my face. And if not those, then my own conscience accuses me. Is Paul blissfully ignorant of these sources of accusation? Likely not, since he is acutely aware of his own past (see 1 Timothy 1:13).
An infinitely more powerful answer to the question of who can accuse us is exactly what Paul says next: “It is God who justifies!”
The substitutionary life of the Son of God in the person of Jesus Christ fulfilled what the law requires of every person ever, both negatively and positively. Jesus was the Lamb of God who bore away the sin of the world. Jesus was that beloved Son with whom God is well pleased. That perfection is accounted to us by the work of the Holy Spirit (John 16:14). The life of Jesus in place of our life is how and why the devil (Satan) was cast out of heaven, no longer able to go before God to accuse us, because the basis of accusation had been crucified.
John referenced this simple yet profound divine response to charges against God’s elect in his first letter: “For if our heart condemns us, God is greater than our heart and knows all things!” (1 John 3:20).