The early Church held closed communion and the Romans spied on them and spread rumors

But why need I disparagingly refer to strange spies and informers, when you allege against us such charges as we certainly do not ourselves divulge with very much noise—either as soon as you hear of them, if we previously show them to you, or after you have yourselves discovered them, if they are for the time concealed from you? For no doubt, when any desire initiation in the mysteries, their custom is first to go to the master or father of the sacred rites.  Then he will say (to the applicant), You must bring an infant, as a guarantee for our rites, to be sacrificed, as well as some bread to be broken and dipped in his blood; you also want candles, and dogs tied together to upset them, and bits of meat to rouse the dogs.  Moreover, a mother too, or a sister, is necessary for you. What, however, is to be said if you have neither? I suppose in that case you could not be a genuine Christian. Now, do let me ask you, Will such things, when reported by strangers, bear to be spread about (as charges against us)? It is impossible for such persons to understand proceedings in which they take no part. The first step of the process is perpetrated with artifice; our feasts and our marriages are invented and detailed by ignorant persons, who had never before heard about Christian mysteries. And though they afterwards cannot help acquiring some knowledge of them, it is even then as having to be administered by others whom they bring on the scene. Besides, how absurd is it that the profane know mysteries which the priest knows not!  They keep them all to themselves, then, and take them for granted; and so these tragedies, (worse than those) of Thyestes or Œdipus, do not at all come forth to light, nor find their way to the public. Even more voracious bites take nothing away from the credit of such as are initiated, whether servants or masters. If, however, none of these allegations can be proved to be true, how incalculable must be esteemed the grandeur (of that religion) which is manifestly not overbalanced even by the burden of these vast atrocities! O ye heathen; who have and deserve our pity, behold, we set before you the promise which our sacred system offers. It guarantees eternal life to such as follow and observe it; on the other hand, it threatens with the eternal punishment of an unending fire those who are profane and hostile; while to both classes alike is preached a resurrection from the dead.

Tertullian
Ad Nationes, Book I
Chapter VII

Not everything is found in Holy Scripture

If, for these and other such rules, you insist upon having positive Scripture injunction, you will find none. Tradition will be held forth to you as the originator of them, custom as their strengthener, and faith as their observer. That reason will support tradition, and custom, and faith, you will either yourself perceive, or learn from some one who has.

…This instances, therefore, will make it sufficiently plain that you can vindicate the keeping of even unwritten tradition established by custom; the proper witness for tradition when demonstrated by long-continued observance.

Tertullian
The Chaplet
Chapter IV

The Real Presence was prefigured in the Old Covenant

When He so earnestly expressed His desire to eat the passover, He considered it His own feast; for it would have been unworthy of God to desire to partake of what was not His own. Then, having taken the bread and given it to His disciples, He made it His own body, by saying, “This is my body,” that is, the figure of my body. A figure, however, there could not have been, unless there were first a veritable body. An empty thing, or phantom, is incapable of a figure. If, however, (as Marcion might say,) He pretended the bread was His body, because He lacked the truth of bodily substance, it follows that He must have given bread for us. It would contribute very well to the support of Marcion’s theory of a phantom body, that bread should have been crucified!  But why call His body bread, and not rather (some other edible thing, say) a melon, which Marcion must have had in lieu of a heart!  He did not understand how ancient was this figure of the body of Christ, who said Himself by Jeremiah: “I was like a lamb or an ox that is brought to the slaughter, and I knew not that they devised a device against me, saying, Let us cast the tree upon His bread,” which means, of course, the cross upon His body. And thus, casting light, as He always did, upon the ancient prophecies, He declared plainly enough what He meant by the bread, when He called the bread His own body. He likewise, when mentioning the cup and making the new testament to be sealed “in His blood,” affirms the reality of His body. For no blood can belong to a body which is not a body of flesh.

…Much more clearly still does the book of Genesis foretell this, when (in the blessing of Judah, out of whose tribe Christ was to come according to the flesh) it even then delineated Christ in the person of that patriarch, saying, “He washed His garments in wine, and His clothes in the blood of grapes”—in His garments and clothes the prophecy pointed out his flesh, and His blood in the wine. Thus did He now consecrate His blood in wine, who then (by the patriarch) used the figure of wine to describe His blood.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XL

Christ receives the prayers of the saints

For as much, then, as there is but one Son of man whose advent is placed between the two issues of catastrophe and promise, it must needs follow that to that one Son of man belong both the judgments upon the nations, and the prayers of the saints.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXIX

Christ saves the entire man — body and soul

Now, since he consists of two parts, body and soul, the point to be inquired into is, in which of these two man would seem to have been lost? If in his body, then it is his body, not his soul, which is lost. What, however, is lost, the Son of man saves. The body, therefore, has the salvation. If, (on the other hand,) it is in his soul that man is lost, salvation is designed for the lost soul; and the body which is not lost is safe. If, (to take the only other supposition,) man is wholly lost, in both his natures, then it necessarily follows that salvation is appointed for the entire man; and then the opinion of the heretics is shivered to pieces, who say that there is no salvation of the flesh. And this affords a confirmation that Christ belongs to the Creator, who followed the Creator in promising the salvation of the whole man.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXVII

Belief in the Virgin Birth is necessary for salvation

Whosoever wishes to see Jesus the Son of David must believe in Him; through the Virgin’s birth. He who will not believe this will not hear from Him the salutation, “Thy faith hath saved thee.” And so he will remain blind, falling into Antithesis after Antithesis, which mutually destroy each other, just as “the blind man leads the blind down into the ditch.”

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXVI

Christ is the only God

For the prayer which He has taught us suits, as we have proved, none but the Creator. It is, of course, another matter if He does not wish to be prayed to, because He is the supremely and spontaneously good God! But who is this good God? There is, He says, “none but one.” It is not as if He had shown us that one of two gods was the supremely good; but He expressly asserts that there is one only good God, who is the only good, because He is the only God.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXVI

Christ as God awards everlasting punishment or salvation

If so, you have here, O heretics, during your present lifetime, a warning that Moses and the prophets declare one only God, the Creator, and His only Christ, and how that both awards of everlasting punishment and eternal salvation rest with Him, the one only God, who kills and who makes alive.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXIV

The souls of the departed live on in an intermediate state before heaven

It must therefore be evident to every man of intelligence who has ever heard of the Elysian fields, that there is some determinate place called Abraham’s bosom, and that it is designed for the reception of the souls of Abraham’s children, even from among the Gentiles (since he is “the father of many nations,” which must be classed amongst his family), and of the same faith as that wherewithal he himself believed God, without the yoke of the law and the sign of circumcision. This region, therefore, I call Abraham’s bosom. Although it is not in heaven, it is yet higher than hell, and is appointed to afford an interval of rest to the souls of the righteous, until the consummation of all things shall complete the resurrection of all men with the “full recompense of their reward.”

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXXIV

Christ is equal to God

Christ. “All things,” He says, “are delivered unto me of my Father.” You may believe Him, if He is the Christ of the Creator to whom all things belong; because the Creator has not delivered to a Son who is less than Himself all things, which He created by Him, that is to say, by His Word.

Tertullian
The Five Books Against Marcion, Book IV
Chapter XXV