Research gathered by Dr. Edgar Foster
Here are some thoughts on Romans 8:16 from various sources.
The Vocabulary of the Greek New Testament (Moulton-Milligan) states:
"SUNMARTUREW, 'bear witness with' (Rom 2:15 al.): cf. BGU I. 86.41ff (A.D. 155), where the signature of each attesting witness is accompanied by the words SUNMARTURW KAI SUNSFRAKIW."
Rogers and Rogers (The New Linguistic and Exegetical Key to the Greek New Testament, page 330) says: "SUMMARTUREI pres. ind. act. SUMMARTUREW (#5210) to bear witness w. someone, to confirm, to testify in support of someone. Used in the papyri where the signature of each attesting witness is accompanied by the words, 'I bear witness w. and I seal w.'" (MM).
"Beareth witness with our spirit [summarturei tōi pneumati hēmōn]. See on Ro 2:15 for this verb with associative instrumental case. See 1Jo 5:10f. for this double witness" (Robertson's Word Pictures).
We also have these words from John Chrysostom:
"Ver. 16. 'The Spirit Itself beareth witness with our spirit, that we are the children of God.'
For it is not from the language merely, he says, that I make my assertion, but from the cause out of which the language has its birth; since it is from the Spirit suggesting it that we so speak. And this in another passage he has put into plainer words, thus: 'God hath sent forth the Spirit of his Son into our hearts, crying, Abba Father.'(Gal. iv. 6.) And what is that, 'Spirit beareth witness with spirit?' The Comforter, he means, with that Gift, which is given unto us. For it is not of the Gift alone that it is the voice, but of the Comforter also who gave the Gift, He Himself having taught us through the Gift so to speak. But when the 'Spirit beareth witness' what farther place for doubtfulness? For if it were a man, or angel, or archangel, or any other such power that promised this, then there might be reason in some doubting. But when it is the Highest Essence that bestoweth this Gift, and 'beareth witness' by the very words He bade us use in prayer, who would doubt any more of our dignity? For not even when the Emperor elects any one, and proclaims in all men's hearing the honor done him, does anybody venture to gainsay."
See the patristic commentary on Romans at http://books.google.com/books?id=yVs2SQkeZEoC&pg=PA211&dq=commentary+on+romans+8:16&sig=cqytt-06pOgp92zUNtmpuhyuYKk
The NET Bible renders Romans 8:16: The Spirit himself bears witness to our spirit that we are God’s children.
The footnote to that texts reads:
Or possibly “with.” ExSyn 160-61, however, notes the following: “At issue, grammatically, is whether the Spirit testifies alongside of our spirit (dat. of association), or whether he testifies to our spirit (indirect object) that we are God’s children. If the former, the one receiving this testimony is unstated (is it God? or believers?). If the latter, the believer receives the testimony and hence is assured of salvation via the inner witness of the Spirit. The first view has the advantage of a σύν- (sun-) prefixed verb, which might be expected to take an accompanying dat. of association (and is supported by NEB, JB, etc.). But there are three reasons why πνεύματι (pneumati) should not be taken as association: (1) Grammatically, a dat. with a σύν- prefixed verb does not necessarily indicate association. This, of course, does not preclude such here, but this fact at least opens up the alternatives in this text. (2) Lexically, though συμμαρτυρέω (summarturew) originally bore an associative idea, it developed in the direction of merely intensifying μαρτυρέω (marturew). This is surely the case in the only other NT text with a dat. (Rom 9:1). (3) Contextually, a dat. of association does not seem to support Paul’s argument: ‘What standing has our spirit in this matter? Of itself it surely has no right at all to testify to our being sons of God’ [C. E. B. Cranfield, Romans [ICC], 1:403]. In sum, Rom 8:16 seems to be secure as a text in which the believer’s assurance of salvation is based on the inner witness of the Spirit. The implications of this for one’s soteriology are profound: The objective data, as helpful as they are, cannot by themselves provide assurance of salvation; the believer also needs (and receives) an existential, ongoing encounter with God’s Spirit in order to gain that familial comfort.