Who will bring any charge against those whom God has chosen? It is God who justifies.
Here's Paul's next question and comfort drawn from our salvation - if it is God who has forgiven us, who can successfully accuse us? God is the judge, it is he who judges. Satan's whole purpose is to condemn us - to cause us to sin and then accuse us of it before God so we will be justly condemned to hell. Yet in Jesus our sins are dealt with - he took the punishment so that there is no need for us to suffer it (assuming we are Christians). He took our sin, he became sinful even though he is God and was perfect, and he suffered God's wrath against sin in our place. God therefore had declared us forgiven - justified. It is as if we had not sinned, we are completely, 100% forgiven if we genuinely follow Christ. Therefore who can accuse? For it is God alone who we have sinned against and he has forgiven us. It is God alone we have offended by disobeying his perfect law, by being out of line from his character, and it is God who made a way, through Jesus, that we might be saved and forgiven in full. If God has chosen to save us, then who can accuse us and condemn us by it? He chooses us and he is more then able and willing to fulfil that by bringing us to heaven eternally.
But, although it is God who chooses, it is also us who need to respond. God is in control, but we are not exempt from responsibility - if we reject him, it is us rejecting him and he will judge justly for that. If we choose hi it is because he first chose us. I've heard God's sovereignty and Human responsibility described as two train tracks (it's not a perfect example, but it might help). They both go the same way, and both are needed. God chooses. We are responsible. I cannot understand it - but that's ok.
As the heavens are higher than the earth,
so are my ways higher than your ways
and my thoughts than your thoughts.
Therefore, have you responded to God? Is this true for you? Or are you still under his condemnation for sin? Jesus offers complete and full salvation - no sin we have committed is to big for him to forgive. His sacrifice is infinite in value. He can save. Will you turn to him? Will you repent and say sorry for your sins, turning around from them and walking towards God as it were by obeying him? Will you put your faith not in your strength or in your knowledge and wisdom, but in him as the creator and saviour, as the only one who can forgive? We cannot save ourselves. We need Jesus.
Note: Obedience to God comes from being saved - it does not save. We could never wipe out our own sin by doing enough good to cover it up. Imagine if a murderer said to the judge that he helped an old lady across the road and was therefore not punished for his crime - that would be unjust. The same applies to us. We have all sinned and God will judge justly. Our sin needs to be dealt with and Jesus alone can deal with it. We obey out of trust in God and thankfulness and genuinely being sorry for sin.
I hope that was helpful! God bless!