But except for this: Romans 5:5
and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us.
I love the visual picture here, God's love is a liquid poured out, gushing, lavishly splashing into our hearts. And that love is the Holy Spirit Himself, who is God. He is said to be almost the forgotten member of the Trinity. The Holy Spirit is introduced in this epistle. And here in Romans he is shown to be a full member of the Triune God and a token of God's love for His people lavishly given to us.
Before salvation: Enmity. War. Rebellion. Struggle. Uncertainty.
After salvation: PEACE
Preacher S. Lewis Johnson said of this passage-
So the apostle’s argument is this. We were enemies. When we were enemies, when we hated God, when we did not want him to minister to us, he came to us, and through the alluring power of the Holy Spirit, he wrought within us, in his own mysterious way, a change of our wills, a change of our disposition, so that the thing that we did not want we ultimately came to want. We, who hated him, by the work of the Holy Spirit, were reconciled as the Holy Spirit brought to us the saving work of the Lord Jesus Christ on the cross. As he says, "When we were enemies, we were reconciled to God through the death of his Son."The Holy Spirit's part in this is that He draws us to Jesus, and He dwells in our hearts to sanctify us, each one of us, throughout our post-salvation lives. What joy.
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Further reading
John MacArthur, article The Ministry of the Holy Spirit
GotQuestions: What does the Holy Spirit do?