The Christians in Rome had ‘received’ the Spirit at conversion. At that time, each believer was given several blessings from the Holy Spirit. Here are some of them.
First, the Spirit convicted him of his sin; second, the Spirit enlightened him about Jesus and his work; third, the Spirit regenerated him and he was enabled to repent of his sins and trust in Jesus; and fourth, the Spirit came to indwell him for ever. Each of these aspects is incredible and it would be good to think about each of them at some stage today.
This leads to consider the ‘how’ of this experience. All new Christians receive the Spirit, but from whom do they receive him? Who sends the Spirit into the hearts of the new believers? The Spirit does not send himself, nor do other Christians through a religious ritual send him into their hearts.
Instead, the Spirit comes to them as a gift from the Father and the Son (as Jesus explained to his disciples in the Upper Room). Each believer has the Spirit, although none of them have the Spirit in his fullness. As Paul says in Ephesians 1, they have the firstfruits of the Spirit.
Receiving the Spirit is the first point that Paul mentions in the quest for assurance that he describes in these verses of Romans 8. The willingness to receive him is evidence of a great change in the heart. Of course, in the joy of the moment of conversion we may not have been conscious of all that was happening to us. Yet we can look back at our conversion and be glad that the Holy Spirit was sent into our hearts.