Yes, I think Robertson's thinking is right on. Here's why. The
doctrinal statement governing the whole epistle is hO QEOS FWS KAI SKOTIA
EN AUTWi OUK ESTIN OUDEMIA (v. 5). The implication of this, as related
to the purpose of the epistle, is succintly summarized both negatively in
v. 6 and positively in v. 7, forming bi-conditionality, BTW.
The amplification of this argument begins in v. 8 and really carries
throughout the epistle. Verse 8, then, parallels v. 6 and gives the
first example of it (saying we have fellowship with God but walking in
darkness - saying we have no sin). Likewise, verse 9 parallels v. 7 and
gives the first example of it (walking in the light - confessing our
sins). Verse 10, of course, parallels both v. 6 and v. 8.
What is the point? Simply that we must interpret v. 9 in parallel with
v. 7. Both verses are conditionals. The point of a conditional is to
establish a necessary relationship between two occurrences or events.
The occurrence of the first (the protasis) necessitates the occurrence of
the latter (the apodosis). Thus, if we walk (customary present) in the
light as He is in the light, then the necessary result (great assurance
here) is we have fellowship with one another and the blood of Jesus His
Son cleanses us from all unrighteousness.
Likewise, in v. 9 if we customarily confess our sins, then the necessary
result is: God is faithful and righteous, and if God is faithful and
righteous, the necessary result (hIVA) is that He will forgive us of all
unrighteousness.
But, what happens if we do not confess our sins? Does this mean God is
not faithful and just? Of course not. As usual, we cannot and must not
infer the negation.
Paul Dixon