Romans 8:13 is a particularly instructive verse. In the Greek, this verse constitutes as a 1st class conditional sentence. The word IF in this verse is the conditional particle (ei), which introduces a protasis of a first class condition that indicates the assumption of truth for the sake of argument. The conditional particle ei, “if” is employed with the indicative mood of the verb zao, “you are living.” Together, they explicitly convey a protasis of a first class condition that indicates the assumption of truth for the sake of argument. In other words assuming the protasis is true (living according to the flesh), then the apodosis is also true (you will spiritually die). It is an IF-Then statement. Paul's warning to the brethren thus conveys - If and let assume brother that it is true for the sake argument you are living in submission to the flesh, i.e. the sin nature.” The apodasis then is you brother will die, i.e. spiritual death.
Verse 14 which follows, affirms that those who are led by the Spirit are sons of God. This actually mirrors the second clause in v.13 that those who live according to the Spirit, will live. If believers choose to live according the spirit, they will live (v.13) and be sons of God because they are led of the Spirit (v.14).
The word "led" in v.14 is a present tense Greek verb. Thus all who are being led are sons of God. One must continue to led by the Spirit in order to be a son of God. If one ceases to be led either through no longer believing or no longer following the Spirit; i.e. disobedience, then one is no longer assured of being a son of God.
Verse 15 states that we have not received a spirit of slavery which leads to fear but instead a spirit of adoption. Contrast this with the unbeliever who only has a spirit of slavery and can only choose to sin because he has not yet been regenerated. Because we have been adopted, we can now choose whether to be slaves to sin or slaves to the Spirit.
Again in verse 16, one must pay attention to the verb tense. The word symmartyrei translated as testifies, bears witness, confirms is also a present tense verb. It describes a present state of ongoing action whereby the Spirit is testifying, is bearing witness, is confirming that we are sons of God. The Greek present tense does not state or even imply that we will remain sons of God in the future. Only at the present, we are continuing to be sons of God. Notice that v.17 does however refer to the future. It states that we are fellow heirs and may also be glorified with him - IF WE SUFFER WITH HIM. Our being glorified is conditional.