“Having refuted the false distinguishing marks, we shall now proceed to present the true distinguishing marks which will manifest themselves always and only in the church, regardless of what her external condition may be. The primary and most eminent distinguishing mark is purity of doctrine — doctrine consistent with the Word of God. We are not now dealing with those who deny fundamental principles, but with those denominated as Christians who acknowledge God’s Word to be the infallible truth. We must therefore consider how God‟s Word defines the distinguishing marks of the true church. Let him depart who does not wish to conduct himself according to God’s Word. However, he who wishes to esteem the Word of God as the only rule of life and doctrine will be able to perceive from this Word that only that church is the true church which has the true doctrine, consistent with the Word.
This is first of all confirmed by such texts where the Word of God is stated 1) to be the means whereby the church is gathered, preserved, and built up; 2) to be a treasure entrusted to her protection; and 3) to be a matter for which she must do battle.
(1) Concerning the first, we read: “Now therefore ye are no more strangers and foreigners, but fellow-citizens with the saints, and of the household of God; and are built upon the foundation of the apostles and prophets, Jesus Christ Himself being the chief corner stone; in whom all the building fitly framed together groweth unto an holy temple in the Lord” (Eph 2:19-21); “That He might sanctify and cleanse it with the washing of water by the Word” (Eph 5:26); “Of His own will begat He us with the Word of truth” (James 1:18); “Seeing ye have purified your souls in obeying the truth” (1 Pet 1:22).
(2) Concerning the fact that the church must protect the Word as a treasure entrusted to her, we read: “… because that unto them were committed the oracles of God” (Rom 3:1-2); “… which is the church of the living God, the pillar and ground of the truth” (1 Tim 3:15).
(3) Finally the church must do battle to maintain purity of doctrine: “It was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints” (Jude 3).
That which we have stated concerning the Word is irrefutably confirmed by the texts mentioned. From these texts it is also evident that these matters are distinguishing marks of the true church, since they are regarded as being characteristic of the church. Inasmuch as a matter is identified by its distinguishing marks, the pure doctrine of God’s Word is therefore a distinguishing mark of the church” (Wilhelmus a’Brakel, The Christian’s Reasonable Service).
This is not a blanket-endorsement of a’Brakel as a true Christian.