Christians believe God became man and died

We who believe that God really lived on earth, and took upon Him the low estate of human form, for the purpose of man’s salvation, are very far from thinking as those do who refuse to believe that God cares for anything.

…Fortunately, however, it is a part of the creed of Christians even to believe that God did die, and yet that He is alive for evermore.

Tertullian
The Five Books Against Marcion, Book II
Chapter XVI

The Spirit of God is God

We must at the outset hold fast the meaning of the Greek scripture, which has afflatus, not spirit. Some interpreters of the Greek, without reflecting on the difference of the words, and careless about their exact meaning, put spirit for afflatus; they thus afford to heretics an opportunity of tarnishing the Spirit of God, that is to say, God Himself, with default.

Tertullian
The Five Books Against Marcion, Book II
Chapter IX

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

Christ’s substance is both man and God

Christ Jesus our Lord (may He bear with me a moment in thus expressing myself!), whosoever He is, of what God soever He is the Son, of what substance soever He is man and God, of what faith soever He is the teacher, of what reward soever He is the Promiser, did, whilst He lived on earth, Himself declare what He was, what He had been, what the Father’s will was which He was administering, what the duty of man was which He was prescribing; (and this declaration He made,) either openly to the people, or privately to His disciples, of whom He had chosen the twelve chief ones to be at His side, and whom He destined to be the teachers of the nations.

Tertullian
Against Marcion
The Prescription Against Heretics
Chapter XX

Christ, the God-man, descended to the dead

Now although Christ is God, yet, being also man, “He died according to the Scriptures,” and “according to the same Scriptures was buried.” With the same law of His being He fully complied, by remaining in Hades in the form and condition of a dead man; nor did He ascend into the heights of heaven before descending into the lower parts of the earth, that He might there make the patriarchs and prophets partakers of Himself. (This being the case), you must suppose Hades to be a subterranean region, and keep at arm’s length those who are too proud to believe that the souls of the faithful deserve a place in the lower regions. These persons, who are “servants above their Lord, and disciples above their Master,” would no doubt spurn to receive the comfort of the resurrection, if they must expect it in Abraham’s bosom. But it was for this purpose, say they, that Christ descended into hell, that we might not ourselves have to descend thither. Well,

Tertullian
A Treatise on the Soul
Chapter LV

Men are not totally depraved. Only Christ was without sin, being himself God.

There is, then, besides the evil which supervenes on the soul from the intervention of the evil spirit, an antecedent, and in a certain sense natural, evil which arises from its corrupt origin. For, as we have said before, the corruption of our nature is another nature having a god and father of its own, namely the author of (that) corruption.  Still there is a portion of good in the soul, of that original, divine, and genuine good, which is its proper nature.  For that which is derived from God is rather obscured than extinguished. It can be obscured, indeed, because it is not God; extinguished, however, it cannot be, because it comes from God. As therefore light, when intercepted by an opaque body, still remains, although it is not apparent, by reason of the interposition of so dense a body; so likewise the good in the soul, being weighed down by the evil, is, owing to the obscuring character thereof, either not seen at all, its light being wholly hidden, or else only a stray beam is there visible where it struggles through by an accidental outlet. Thus some men are very bad, and some very good; but yet the souls of all form but one genus: even in the worst there is something good, and in the best there is something bad. For God alone is without sin; and the only man without sin is Christ, since Christ is also God.

Tertullian
A Treatise on the Soul
Chapter XLI

Christ is to all God and Lord

But Christ’s Name is extending everywhere, believed everywhere, worshipped by all the above-enumerated nations, reigning everywhere, adored everywhere, conferred equally everywhere upon all. No king, with Him, finds greater favor, no barbarian lesser joy; no dignities or pedigrees enjoy distinctions of merit; to all He is equal, to all King, to all Judge, to all “God and Lord.” Nor would you hesitate to believe what we asseverate, since you see it taking place.

Tertullian
An Answer to the Jews
Chapter VII

Deity was Christ’s essential possession

Surely Christ, then, had a right to reveal Deity, which was in fact His own essential possession, not with the object of bringing boors and savages by the dread of multitudinous gods, whose favour must be won into some civilization, as was the case with Numa; but as one who aimed to enlighten men already civilized, and under illusions from their very culture, that they might come to the knowledge of the truth. Search, then, and see if that divinity of Christ be true. If it be of such a nature that the acceptance of it transforms a man, and makes him truly good, there is implied in that the duty of renouncing what is opposed to it as false; especially and on every ground that which, hiding itself under the names and images of dead, the labors to convince men of its divinity by certain signs, and miracles, and oracles.

Tertullian
Apology
Chapter XXI