35-1. All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The accused party may be allowed, but shall not be compelled, to testify; but the accuser shall be required to testify, on the demand of the accused. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.

DIGEST : 1976 [M4GA, 4-66, F.5, p. 102] - adopted? This text is interesting for its requirement of belief in God and in a future state of reward and punishments. Such requirement is not found in PCUSA texts until the revision of 1858 and the requirement subsequently finds its way into the respective revisions of both the northern and southern Churches. Virtually the same wording is also found in the UPCNA text of 1867, and it is probably not too surprising that the UPCNA might have adopted portions of the PCUSA revision of 1858.
For one possible explanation of how this provision might have come to be incorporated into the text, I recently came across a book by D.X. Junkin [1808-1888], titled The Oath A Divine Ordinance. Junkin, a prominent Presbyterian pastor from a notable Presbyterian family, makes much the same point on the necessity of a belief in a future state of rewards and punishments [see below under Commentary]. It seems plausible that Junkin may have influenced the 1858 revision committee, or may have otherwise contributed to a larger discussion in mid-19th century Presbyterian thought. Hopefully further research will shed some light on this theory.

BACKGROUND & COMPARISON :
PCA 1973, 9-1, Adopted text, as printed in the Minutes of General Assembly, p. 149
Continuing Presbyterian Church 1973, 9-1, Proposed text, p. 48
All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The accused party may be allowed, but shall not be compelled, to testify; but the accuser shall be required to testify, on the demand of the accused. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.

PCUS 1879, IX-1
All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The accused party may be allowed, but shall not be compelled, to testify; but the accuser shall be required to testify on the demand of the accused. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.

PCUS 1869 draft, IX-1
All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The parties may be allowed, but shall not be compelled, to testify. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.

PCUS 1867 draft, IX-1
All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The parties may be allowed, but shall not be compelled, to testify. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.

OTHER COMPARISONS :
PCUSA, 1820, Revised Form of Government and Forms of Process, Book II, VI-I., VI-II., & VI-III.
I. Judicatories ought to be very careful and impartial in receiving testimony. All persons are not competent as witnesses; and all who are competent are not credible.
II. A competent witness is one who ought to be admitted and heard. The competency of a witness may be affected by his want of the proper age; by a want of any of the senses essential to a knowledge of the matter which he is called to establish; by weakness of understanding; by infamy of character; by being under church censure for falsehood or perjury; by nearness of relationship to any of the parties; and by a variety of considerations which cannot be specifed in detail.
III. Where there is room for doubt with regard to any of these points, either party has a right to challenge witnesses; and the judicatory shall candidly attend to the exceptions, and decide upon them.

PCUSA, 1858 draft, VII-2
All persons, whether parties or otherwise, are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. Either party has a right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency.

UPCNA 1867, 7-3
All persons, whether parties or otherwise, are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments, or have not sufficient intelligence to understand the obligation of an oath. Either party has a right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide on his competency.

OPC, 1988-2005, IV.A.4.a [unchanged in OPC 2011, BoD IV.A.4.a.]
Any person may be a witness in a judicial case if the trial judicatory is satisfied that he has sufficient competence to make the affirmation required of witnesses in this Chapter, Section A, 4, b.
and
OPC, 1988-2005, IV.B.1 and 2
1. Evidence must be factual in nature. It may be direct or circumstantial. Caution should be exercised in giving weight to evidence which is purely circumstantial.
2. The accused may object to the competency of any witness and the authenticity, admissibility, and relevancy of any testimony or evidence produced in support of the charge and specifications. The trial judicatory shall decide on all such objections after allowing the accused to be heard in support thereof. [unchanged in OPC 2011, BoD IV.B.1 and 2.]
and
OPC, 1988-2005, IV.B.7
7. All questions concerning the competency of any witness and the authenticity, admissibility, and relevancy of any testimony or evidence taken by a commission shall be determined by the trial judicatory after the accused has been given the opportunity to be heard. [unchanged in OPC 2011, BoD IV.B.7.]

ARP, 1958, V-§2ff.
2. Every court shall be its own judge as to who shall be admitted as witnesses in a case. Either party has the right to challenge any witness that may be called to the stand, giving his reasons for the challenge, and the court shall decide whether the witness shall be allowed to testify.
3. The credibility of witnesses, or the degree of credit to be given to their testimony, may be affected by relationship to either of the parties, by interest in the result, by want of proper age, by weakness of understanding, by defect in any of the senses, by enmity to the accused, by personal character, and by various other circumstances to which the court should carefully attend and for which it should make due allowance in its decision.
4. Private writings and printed publications, the genuineness and authorship of which are clearly established, shall be received as evidence.
7. Hearsay evidence is to be received only where it is impossible to obtain the evidence directly, and even then with great care and discrimination.
9. Circumstantial evidence may be received either to corroborate positive testimony, or as conclusive when it is of such character to produce full conviction on the mind of the court.”

COMMENTARY :
D.X. Junkin, The Oath A Divine Ordinance (1845), p. 11-12
But the most perfect laws are inefficient without sanctions : that is--without some reward, penalty, or other condition affixed, as a motive to obedience. Even perfect man in Eden was placed under a constitution and laws to which a penal sanction was affixed :--"In the day thou eatest thereof thou shalt surely die." And if Adam in Paradise needed motives to obedience--much more do his fallen descendants. Indeed, the necessity for adequate sanctions to law, arises from the very nature of all subjects of law. None are proper subjects of law except voluntary agents. A being who has no will, is not an accountable being, and is not a proper subject of moral law. But the will is determined by motives ; and the motive gives character to the action. He that acts, from good motives, in a right way, acts well. He that acts from bad motives, in any way, does wrong. Man will not act at all without motive ; such is his constitution. And if the law presents no motive to obedience, it cannot secure obedience from voluntary agents.
Reward and punishment are relative ideas. Each indicates the possibility of the other. Nothing can be a reward that does not augment the happiness of him upon whom it is bestowed. If, then, the reward be withheld, he is less happy than he would be with it : i.e., he is punished. To punish is to make less happy--to reward is to make more happy. The ideas are correlative ; and by consequence, where reward of obedience is affixed as the sanction of law, punishment of disobedience is implied ; and vice versa.
Now that these two ideas are not only promulgated, but that they are often reiterated in the word of God, no reader of that volume can fail to perceive. It reveals a system of rewards and punishments, addressing the hopes and fears of men, in relation both to time and eternity. It authorizes the employment of rewards and punishments by the civil magistrate. It promises that in some cases rewards or punishments shall be administered in this life, in the providential dispensations of God. But it appeals, with the most efficient power, to human hopes and fears, by bringing "immortality to light,"--by pointing to "a judgment to come,"--and by disclosing the glories of a Heaven, and the horrors of a hell ! It thus teaches, with an eloquence only less terrible than the dread reality, that "the wages of sin is death." Thus it appears that the Bible furnishes both laws and sanctions. It supplies the only efficient motives to obedience."

F.P. Ramsay, Exposition of the Book of Church Order (1898, pp. 215-216), on RoD IX-1
CHAPTER IX. - OF EVIDENCE.
Paragraph 1 shows who are competent witnesses, but paragraph 2 exempts husband and wife from testifying against each other. Paragraph 3 lays down the rule requiring corroborative evidence, and paragraph 4 contains a regulation that aims to make witnesses independent of each other. Paragraph 5 prescribes the method of examining witnesses ; 6, the form of oath or affirmation ; and 7, the method of recording the testimony. Paragraph 8 shows how the records of one court are to be authenticated to another ; paragraph 9, the value of such authenticated testimony ; and paragraph 10, how testimony may be taken in the absence of the court. Paragraph 11 secures the use of the members of the court as witnesses, and paragraph 12 the use of all church officers and members as witnesses. And paragraph 13 regulates the use of new evidence after trial, and paragraph 14 after appeal.
206.--I. All persons of proper age and intelligence are competent witnesses, except such as do not believe in the existence of God, or a future state of rewards and punishments. The accused party may be allowed, but shall not be compelled, to testify; but the accuser shall be required to testify on the demand of the accused. Either party has the right to challenge a witness whom he believes to be incompetent, and the court shall examine and decide upon his competency. It belongs to the court to judge of the degree of credibility to be attached to all evidence.
Accuser here must be interpreted to mean the prosecutor. The only grounds of challenge of a witness are too great youth, too little intelligence (i.e., ability), lack of belief in God, or lack of belief in a future state of rewards and punishments. The accused cannot be debarred from testifying.

CONSTITUTIONAL INQUIRY :

JUDICIAL :
Points touching BCO 35-1 appear in the complaint of TE Eric Dye, et al., vs. Missouri Presbytery, M14GA (1986), 14-89, p. 212 [or PCA Digest, Judicial Case #56, pp. 354-360]