First, regarding the “bond of union among Christians.”
Passing by, for the present, the various stupid schemes, all different and all wrong, pursued by Roman Catholics, Socinians, Arians, Covenanters, Seceders, Presbyterians, High-Churchmen, Baptists, Independents, and so forth, let us attend to the plan of teaching the truth pursued by God–by the Lord Jesus Christ–by the Holy Spirit, in presenting it to all men in the scriptures, and by the apostles and all who first preached it–a plan founded in the very nature of the saving truth itself, and into which ignorant missionaries feel themselves driven when every human scheme has failed. But what is the truth? Times out of number we are told in scripture that the grand saving truth is, that “Jesus is the Christ.” This is the bond of union among Christians–the essence–the spirit of all revelation. All the scriptures testify and confirm this simple truth, that “he that believeth that Jesus is the Christ, is begotten by God.” John v. 2. For he who believeth it, sets to his seal that God is true. Such a one, John says, loveth God and Christ and the brethren, keepeth his commands, and is purified from all his sins, and overcometh the world, and shall be saved. Christ declared when departing into heaven, that he that believeth not shall be damned.
A major contributor to The Christian Baptist was Walter Scott, the first great missionary of the Restoration Movement. He wrote,
The first of these prefatory articles is, that the members of a church of Christ are united to one another by the belief of a matter of fact, viz. that “Jesus is the Christ, the Son of God,” and not by any attribute of government, catholic or sectarian. The second is, that the scriptures propose the belief of this fact, that “Jesus is the Christ,” as the only means for increasing the body or church of God. …
But that the glorious truth, and nothing else, holds the saints together in particular churches, is evident from the holy epistles which are addressed to them in their individual capacities. … These things may suffice to show that the bond of union among christians is the belief of a matter of fact, viz. that Jesus of Nazareth is the Son of God.
I think, and perhaps, too, the reader thinks, that in my last paper it was showed by a series of New Testament quotations, that this peerless fact, that “Jesus is the Christ,” forms the sole bond of union among the holy brethren, and is also the means through faith for increasing the body of Christ in the earth.
What were the boundary markers of the church of Christ, according to Campbell and Scott?
Well, there was just one. “Jesus is the Christ.”
What distinguished his followers from other believers?
The fact that they accepted “Jesus is the Christ” as an entirely sufficient bond of union.
Did the boundary markers move just a little from 1823 to now?
PS — The church planters among us may enjoy this one from Campbell —
The christian religion is a social religion, and cannot be exhibited to the full conviction of the world, only when it appears in this social character. An individual or two, in a pagan land, may talk about the christian religion, and may exhibit its morality as far as respects mankind in general; but it is impossible to give a clear, a satisfactory, a convincing exhibition of it, in any other way than by exhibiting a church, not on paper, but in actual existence and operation, as divinely appointed. … Then suppose a christian church were to be placed on the confines of a heathen land, as some of them must inevitably be, the darkness of paganism will serve, as a shade in a picture, to exhibit the lustre of christianity. Then the heathen around them will see their humility; their heavenly-mindedness, their hatred of garments spotted with the flesh, their purity, their chastity, their temperance, their sobriety, their brotherly love; they will observe the order of their worship, and will fall down in their assemblies, as Paul affirms, and declare that God is in them of a truth. Then will be verified anew the words of the Saviour–“If ye love one another, all men will know that you are the disciples of the Saviour of the world.” They will say to one another, and proclaim to their countrymen on every occasion, “These christians are peaceful, benevolent, humane, forgetful, and forgiving of injuries; they hate war, oppression, theft, falsehood, detraction; they are always talking of the hope of a glorious resurrection from the dead, and are looking for the coming of him whom they call their Lord. In their assemblies there is order, peace, love, and harmony. Their chief guide is not distinguished by his dress, as our priests, nor does he, like them, live upon the sweat and sacrifices of the people. He works with his own hands as those who meet with him in their assembly. They repay the curses of wicked pagans with blessings, and their benevolence is not confined to themselves. They are as benevolent to all our people as to themselves–come, see if their religion is not better than ours–better than all others.” When the christian church assumes such a character, there will be no need of missionaries. She will shine forth in the doctrine and in the practice of her members, as the sun in the firmament, and the brightness of her radiance will cheer the region and shadow of death.