16 The Spirit Himself bears witness with our spirit that we are children of God,
There is a contrast between verse 15, where we give testimony to our new relationship with God, and verse 16, where the Holy Spirit bears witness with us.
Verse 16 shows how it is possible that believers can confidently reach out to God as their Father (v.14). The Holy Spirit confirms in our hearts by bearing witness with our spirit that we are God’s children.
16 The Spirit Himself [emphatic] bears witness [joint-witness] with our spirit
The Spirit of God bears witness to God along with our spirit’s witness about our salvation. This is double testimony that we are God’s children. The double witness is true, independent of our emotions or subjectivity. The Spirit does not bear witness “to” our spirit but “with” our spirit. The Holy Spirit along with our spirit establishes something as true. A biblical principle is that the presence of two witnesses establishes a fact.
When a Christian prays, he does not pray alone. The Holy Spirit enters our prayer as well to enable us to pray effectively. No one less than the Spirit Himself bears witness with our “spirit.” The phrase “bears witness with” comes from two words in the Greek: to testify and with. The Holy Spirit testifies together with Christians about the assurance of their salvation. The phrase “with our spirit” indicates that both the Holy Spirit and our spirit witness that we are God’s children. We are not unaided in this, for the Holy Spirit directly gets involved with our prayer.
that we are children of God,
The Holy Spirit bears joint-testimony with us that we are the children of God. Not only are believers “sons” of God, but we are the “children of God.” We are related to God by spiritual birth. We were once in an alien family separated from God, but now we are children who know our Father.
There is an agreement between the Holy Spirit and our human spirit that we are God’s children. This is separate from and higher than something that comes subjectively from within us. It is certitude of the Holy Spirit’s within our lives. On this direct testimony from the Holy Spirit rests the certainty of our convictions. God welcomes and assures us as a Parent in His family.
The Christian has a double ground of assurance about his salvation.
The Holy Spirit enables Christians to see themselves as true sons of God. The Spirit gives concurrent testimony with us that we have status with God. He brings to our conscience the certainty of God’s promises in His Word. God has spoken or declared by promise that we can know for a fact that we are Christians.