One of the distinguishing marks of a true Christian is that the Holy Spirit indwells him or her. Paul reminds the Ephesians (1:13-14) that the Holy Spirit functions as a seal of ownership (a seal was a mark that indicated ownership) and as an earnest of our future inheritance (he is the guarantee and sample of the future life that will be enjoyed). John here in verse 13 tells his readers that the presence of the Spirit is the proof that they are abiding in Christ and he in them.
This of course raises a question, How do we know that we have the Spirit? John gives other evidences of the presence of the Spirit in the verses that follow verse 13. In verses 14 to 16 he refers to the Spirit’s role in enabling Christians to witness on behalf of Christ.
John reminds his readers that they understood the plan of salvation, ‘that the Father sent his Son to be the Saviour of the world.’ This kind of understanding is not the same as merely knowing about another person – John does not have in mind mere knowledge of facts. Instead, John is stressing three important details of Christian witness.
First, John writes in such a way that he assumes that all Christians have experienced the grace of God and bear witness to it. This is what he means when he says that they have seen and testify to it. Their experience was so wonderful that they could not keep silent about it. They had discovered the way of salvation and they wanted others to know it as well.
Second, John mentions also that they defended the Saviour’s name. There were people who were denying that Jesus was the Son of God. When these Christians heard such denials, they spoke up and confessed that he was the divine Son of God. Those who were denying may have been willing to give Jesus a prominent place, but that was not enough for his true followers. They made it very clear to detractors that Jesus was God’s eternal Son. The reason why they did so was because the Holy Spirit was working in their hearts.
Third, what is the consequence of experiencing the help of the Spirit in this matter of faithfully witnessing to Jesus? John tells us in verse 16 – we have assurance of God’s love. Note the order of John’s description – ‘coming to know’ precedes ‘believing’ the love God has for us. We would expect John to have them the other way round. Yet our faith is always strengthened by experience. The Spirit gives special assurance to faithful believers and thereby their faith is strengthened.