Wherefore - In consequence of this privilege of addressing God as your Father.
Thou art no more - You who are Christians.
A servant - In the servitude of sin; or treated as a servant by being bound under the oppressive rites and ceremonies of the Law; compare the note at Galatians 4:3
But a son - A child of God, adopted into his family, and to be treated as a son.
And if a son ... - Entitled to all the privileges of a son, and of course to be regarded as an heir through the Redeemer, and with him. See the sentiment here expressed explained in the the note at Romans 8:17.