The Holy Spirit Himself bears testimony [along] with our [human] spirit that we are the children of God.
He has placed His seal of ownership upon us by giving us the down payment of the Holy Spirit [to live] in our hearts.
Now God is the One who has prepared us for this very thing [i.e., the receiving of a new body to live in]. He has [also] given us the Holy Spirit as a down payment. [Note: This refers to the gift of the indwelling Holy Spirit as being a guarantee that we will receive the balance of our inheritance in the form of a new body].
You [Gentiles] also, who are in Christ, after having heard the message of truth, the good news about your salvation, and [after] believing in Him, were sealed [i.e., as evidence of ownership] with the promised Holy Spirit. [or, "the Holy Spirit who brings promises"].
For this is what we boast of: Our conscience testifies that we have conducted ourselves in the world, and especially toward you, in holiness and godly sincerity, not with human wisdom, but with the favor of God.
And do not grieve God's Holy Spirit by whom you were sealed [i.e., given evidence of God's ownership] until the day of redemption [i.e., until you receive your new body in heaven. See 1:14].
And this is how we will know that we are following [God's] truth, and will have assurance [of it] in our hearts and before God [as well].
This is [one way] to know we live in [fellowship with] God and that He lives in our hearts: [It is] because He has given us His Holy Spirit [to live within us, See 3:24].
The person who [sincerely] believes in God's Son has the evidence [of it] within his heart, [confirming God's testimony]. The person who does not believe God [i.e., what He says about Jesus], accuses Him of lying, because he has not accepted the [irrefutable] testimony of God concerning [the identity and mission of] His Son.
And not just the world, but even we [Christians] ourselves, who have the firstfruits of the Holy Spirit [i.e., the first installment of our inheritance from God] also groan within us, eagerly waiting to be adopted as [God's] children, and receive back our bodies [i.e., in the resurrection].
And in the same way, the Holy Spirit helps [us in] our weakness; for we do not know what we ought to pray for [Note: Some translators understand this as "how to pray"]. But the Holy Spirit goes to God on our behalf with groanings [in our hearts] that are not expressed [in words].