Next verse. 6 And because you
ARE sons, God has sent forth the Spirit of His Son into your hearts, crying out, [
b]“Abba, Father!” 7 Therefore you are no longer a slave but a son, and if a son, then an heir [
c]of God [
d]through Christ.
This is talking about sonship in the context of ADOPTION, and the very next verse states it as a present reality.
It's just like we ALREADY HAVE eternal life, but have not entered into the full expression of it yet, which will come at the glorification.