Baptism forgives sins, delivers from death and regenerates man

Oh, what a god is this! everywhere perverse; nowhere rational; in all cases vain; and therefore a nonentity!—in whose state, and condition, and nature, and every appointment, I see no coherence and consistency; no, not even in the very sacrament of his faith! For what end does baptism serve, according to him? If the remission of sins, how will he make it evident that he remits sins, when he affords no evidence that he retains them? Because he would retain them, if he performed the functions of a judge. If deliverance from death, how could he deliver from death, who has not delivered to death? For he must have delivered the sinner to death, if he had from the beginning condemned sin. If the regeneration of man, how can he regenerate, who has never generated? For the repetition of an act is impossible to him, by whom nothing any time has been ever done. If the bestowal of the Holy Ghost, how will he bestow the Spirit, who did not at first impart the life? For the life is in a sense the supplement of the Spirit. He therefore seals man, who had never been unsealed in respect of him; washes man, who had never been defiled so far as he was concerned; and into this sacrament of salvation wholly plunges that flesh which is beyond the pale of salvation!

Tertullian
The Five Books Against Marion, Book I
Chapter XXVIII

Salvation can be lost

And what will it be to be cast away, but to lose that which a man was in the way of obtaining, were it not for his rejection—that is, his salvation?  Therefore his being cast away will involve the forfeiture of salvation; and this sentence cannot possibly be passed upon him, except by an angry and offended authority, who is also the punisher of sin—that is, by a judge.

Tertullian
The Five Books Against Marcion, Book I
Chapter XXVIII

The doctrine of God came from the Apostles

Some disputed about eating idol sacrifices, others about the veiled dress of women, others again about marriage and divorce, and some even about the hope of the resurrection; but about God no one disputed. Now, if this question also had entered into dispute, surely it would be found in the apostle, and that too as a great and vital point. No doubt, after the time of the apostles, the truth respecting the belief of God suffered corruption, but it is equally certain that during the life of the apostles their teaching on this great article did not suffer at all; so that no other teaching will have the right of being received as apostolic than that which is at the present day proclaimed in the churches of apostolic foundation.

…Show us, then, one of your churches, tracing its descent from an apostle, and you will have gained the day.

Tertullian
The Five Books Against Marcion, Book I
Chapter XXI

The sacraments show God’s purpose for the physical world

Indeed, up to the present time, he has not disdained the water which the Creator made wherewith he washes his people; nor the oil with which he anoints them; nor that union of honey and milk wherewithal he gives them the nourishment of children; nor the bread by which he represents his own proper body, thus requiring in his very sacraments the “beggarly elements” of the Creator.

Tertullian
The Five Books Against Marcion, Book I
Chapter XIV

Creation is the evidence for God itself

Never shall God be hidden, never shall God be wanting. Always shall He be understood, always be heard, nay even seen, in whatsoever way He shall wish. God has for His witnesses this whole being of ours, and this universe wherein we dwell.  He is thus, because not unknown, proved to be both God and the only One, although another still tries hard to make out his claim.

Tertullian
The Five Books Against Marcion, Book I
Chapter X

God is the unbegotten maker of all things

It is not, therefore, for the name of god, for its sound or its written form, that I am claiming the supremacy in the Creator, but for the essence to which the name belongs; and when I find that essence alone is unbegotten and unmade—alone eternal, and the maker of all things—it is not to its name, but its state, not to its designation, but its condition, that I ascribe and appropriate the attribute of the supremacy.

Tertullian
Five Books Against Marcion, Book I
Chapter VII

God is one

But the Christian verity has distinctly declared this principle, “God is not, if He is not one;” because we more properly believe that that has no existence which is not as it ought to be. In order, however, that you may know that God is one, ask what God is, and you will find Him to be not otherwise than one. So far as a human being can form a definition of God, I adduce one which the conscience of all men will also acknowledge,—that God is the great Supreme existing in eternity, unbegotten, unmade without beginning, without end.

…Whatever other god, then, you may introduce, you will at least be unable to maintain his divinity under any other guise, than by ascribing to him too the property of Godhead—both eternity and supremacy over all. How, therefore, can two great Supremes co-exist, when this is the attribute of the Supreme Being, to have no equal,—an attribute which belongs to One alone, and can by no means exist in two?

Tertullian
Five Books Against Marcion, Book I
Chapter III

The definition of a heretic

…a heretic may from his case be designated as one who, forsaking that which was prior, afterwards chose out for himself that which was not in times past. For in as far as what was delivered in times past and from the beginning will be held as truth, in so far will that be accounted heresy which is brought in later.

Tertullian
Five Books Against Marcion, Book I
Chapter I