If you MISUSE Campbell to encourage people NOT to be baptized then Campbell places you outside of the Pale.

We do not suppose all unimmersed persons to be absolute aliens from the family of God–nor are they absolutely excluded from any participation with us in prayer or in the Lord's supper. (Alexander Campbell, "The Christian Magazine," MH, March, 1845)

To the idea that Campbell repudiated his views in the Lunenburg Letter, this is eight years after he wrote the response to the Lunenburg Letter.

About this period [1847] the "Evangelical Alliance," designed to promote the union of Christians, attracted much of Mr. Campbell's attention, and was hailed by him with great satisfaction as an indication of the approach of a better era. As to the basis of union which it proposed, he expressed a substantial agreement, though objecting to some of the expressions employed as unscriptural. He pointed out the resemblance of the movement to that of the "Christian Association" in Washington in 1809, which, like the Evangelical Alliance, assumed not the character of a church, but of a society to promote union among Christians, and remarked, at the close of his article:

”I said at the beginning, I say at the close, of my notice of the Evangelical Alliance, that I thank God and take courage at every effort, however imperfect it may be, to open the eyes of the community to the impotency and wickedness of schism, and to impress upon the conscientious and benevolent portion of the Christian profession the excellency, the beauty and the necessity of co-operation in the cause of Christ as prerequisite to the diffusion of Christianity throughout the nations of the earth.

”The Reformation for which we plead grew out of a conviction of the enormous evils of schism and partyism, and the first article ever printed by any of the co-operants in the present effort was upon the subject of the necessity, practicability and excellency of Christian union and communion, in order to the purification and extension of the Christian profession. The abjuration of human creeds as roots of bitterness and apples of discord, as the permanent causes of all sectarianism, was set forth as a preliminary step to the purification of the Church and the conversion of the world. The restoration of a pure speech, or the giving of Bible names to Bible ideas, followed in its train, and from these standing-points we have been led step by step to our present position, each one of the prime movers adding to the common stock something of importance, until matters have issued in one of the most extensive moral and ecclesiastical movements and revolutions of the present age.”

(A. Campbell's views on the Evangelical Alliance, from Robert Richardson’s Memoirs of Alexander Campbell, Volume II, Chapter XVII) Campbell could hardly work for "Christian" unity across denominational lines if he considered other denominations non-Christians. He could only refer to denominational differences as "schisms" because he saw the denominations as being of "common stock" (same family).

2. And in the second place, why should we so often have quoted and applied to apostate Christendom what the Spirit saith to saints in Babylon–"Come out of her, my people, that you partake not of her sins, and that you receive not of her plagues"–had we imagined that the Lord had no people beyond the pale of our communion!

"None of you [Christians] have ever yet attempted to show how Christians can be united on your principles.
You have often showed how they may be divided, and how each party may hold its own, but while you pray for the visible unity of the disciples, and advocate their visible disunity, we cannot understand you." March, 1827, vol. 4.

OF THE BAPTIST'S BAPTISM:

Besides, immersion gives a constitutional right of citizenship in the universal kingdom of Jesus;
whereas with our opponents, themselves being judges, their "baptism" gives the rights of citizenship
only in some provinces of that kingdom.

That is why a Church of Christ type would not be accepted if they believed Acts 2:38 as written. In my 79ish years one would be asked what they understood about their baptism to make sure that they are happy.

It seems that A. Campbell received a lot of flack for writing that response that was quoted by Royce. He tried to retract (explain) what he said on several occasions shortly thereafter.

He seems to use adult immersion for the forgiveness of sins as a line for fellowship: “baptism: for the sake of union among Christians, it may be easily shown to be the duty of all believers to be immersed”.

Furthermore he states:
“The case is this: When I see a person who would die for Christ; whose brotherly kindness, sympathy, and active benevolence know no bounds but his circumstances; … because he never happened to inquire, but always took it for granted that he had been scripturally baptized; … I feel no disposition to flatter such a one; but rather to disabuse him of his error.”

He does seem to understand that God may apply his grace to the ignorant and thus Campbell left open other possibilities. But that should not give anyone a false confidence, “Now, in our judgment, there is not on earth a person who can have as full an assurance of justification or of remission of sins, as the person who has believed, confessed his faith, and been intelligently buried and raised with the Lord."

With that said, Campbell made it clear that anyone not immersed for forgiveness of sins is in error.

All quotes from The Millennial Harbinger, December 1837, pp 564-565 as reprinted by College Press.
This is available on Google Books.

I don't base my understanding of the Bible on what Campbell or others say. But they are good reference.

For many years, there's been considerable disinformation spread about Alexander and Thomas Campbell in an effort to make them fit the 20th Century doctrinal mold. I've been told in the last month that Alexander Campbell retreated from his position stated in the Lunenburg letter later. He never did. Nor did he retreat from the Richmond correspondence where he insisted that Baptists not be rebaptized.

Some have argued that his support for the missionary society was the product of old age and sickness, but he campaigned vigorously for a national missionary society. The truly telling fact is that his church was part of an association of churches that sent Walter Scott out as the Restoration Movement's first missionary — while he was still quite young. It was during this mission campaign that Scott began preaching the 5-finger plan of salvation to great effect.

The best resource is Leroy Garrett's The Stone-Campbell Movement, which is very well written and very well documented. Also excellent is James Deforest Murch's Christians Only, written from the Christian Church perspective. These are by far the two best and most honest treatments of our history.

Again, ALEXANDER CAMPBELL wrote: “The case is this: When I see a person who would die for Christ: whose brotherly kindness, sympathy, and active benevolence knows no bounds but his circumstances: whose seat in the Christian assembly is never empty; whose inward piety and devotion are attested by punctual obedience to every known duty; whose family is educated in the fear of the Lord; whose constant companion is the Bible; I say, when I see such a one ranked amongst heathen men and publicans, because he never happened to inquire, but always took it for granted that he had been scripturally baptized, and that [ranking] too, by one greatly destitute of all these public and private virtues, whose chief or exclusive recommendation is that he has been immersed, and that he holds a scriptural theory of the gospel, I feel no disposition to flatter such a one, but rather to disabuse him of his error. And while I would not lead the most excellent professor in any sect to disparage the least of all the commandments of Jesus, I would say to my immersed brother as Paul said to his Jewish brother who gloried in a system which he did not adorn: ‘Sir, will not his uncircumcision, or unbaptism, be counted to him for baptism? and will he not condemn you, who, though having the literal and true baptism, yet dost transgress or neglect the statues of your King?’” (Millennial Harbinger, 1837, p. 565.)

BARTON W. STONE wrote: “My opinion is that immersion is the only baptism. But shall I therefore make my opinion a term of Christian fellowship? If in this case I thus act, where shall I cease from making my opinions terms of fellowship? I confess I see no end. . . . Let us still acknowledge all to be brethren, who believe in the Lord Jesus, and humbly and honestly obey him, as far as they know his will, and their duty.” (Christian Messenger, 1831, p. 19, 21.)

WALTER SCOTT wrote: “Christians who have not been baptized for the remission of their sins! Strange! Whoever read of such Christians in God’s Word? But the times are peculiar, and as faith does purify the life of a man, and as the man of pure life and pure heart is accepted of God and may receive the Spirit, therefore we must allow, that there are now a days Christians in heart and life who have not been baptized for the remission of their sins. What evidences, then, have they for themselves and others, that they are possessed of the Spirit? None but the moral graces which have already been quoted, viz: love, joy etc.; they don’t need to depend upon an opinion; they feel within themselves and show to those without them by their fruits, that they have been made partakers of the Spirit of Christ.” (The Evangelist, No. 2, Vol. 2, Feb 4, 1833, p. 49.)

Well, there you go.
Royce

Please help me understand. You are saying that Campbell never believed that adult baptism was required for forgiveness of sins and thus also not required to enter heaven?

I'm referring to what Thomas Campbell wrote in the Declaration and Address. Introducing the thirteen propositions for unity, he said:

Let none imagine that the subjoined propositions are at all intended as an overture towards a new creed, or standard, for the church; or, as in any wise designed to be made a term of communion;–nothing can be farther from our intention.

That applied to all thirteen propositions, in particular to this statement in proposition eleven:

Lastly, that in all their administrations they keep close by the observance of all divine ordinances, after the example of the primitive church, exhibited in the New Testament; without any additions whatsoever of human opinions or inventions of men.

Thomas Campbell clearly did not intend for that to become"a new creed, or standard, for the church" nor "a term of communion." For several decades that was the understanding that guided the Restoration Movement. But an obvious change occurred somewhere around the middle of the 1800's.