In order to answer this question we have to ask ourselves: What is religion? I'll keep it brief, because this could get long and ugly fast. James Rives describes it as: "A conception of, reverence for, and desire to please or live in harmony with some superhuman force, as expressed through specific beliefs, principles, or actions." It could also answer questions that are metaphysical, material, spiritual, or even human in nature. Religion should also incorporate a coherent system of beliefs, usually (but not always!) accompanied by: a moral code, sacred scriptures/texts, professional clergy/priests, and distinctive practices and customs of worship.

The problem we, with our Judeo-Christian/Abrahamic glasses, encounter has to do with the sheer variety of Roman religion. It's incredibly varied, and that diversity presents what we perceive as an incoherence. To us there seems to be a lack of canonical texts and dogma. But the words the Romans used to describe religion do meet the criteria of our (incredibly brief) definition of religion above. Namely:

• sacer - holy, consecrated, hallowed, of, or relating to, the gods
• pietas - proper respect extended toward the divine
• cultus - honor, reverence, adoration
• religio - this word "becomes" what we define as "religion" in the hands of Christian authors, such as Tertullian, who says, "This whole confession of these beings, in which they declare that they are not gods, and in which they tell you that there is no God but one, the God whom we adore, is quite sufficient to clear us from the crime of treason, chiefly against the Roman religions (religiones). For if it is certain the gods have no existence, there is no religion in the case. If there is no religion (religio), because there are no gods, we are assuredly not guilty of any offense against religion (religio). Instead of that, the charge recoils on your own head: worshipping a lie, you are really guilty of the crime you charge on us, not merely by refusing the true religion (veram religionem) of the true God, but by going the further length of persecuting it" (Tertullian, Apol. 24).

Moreover, the Romans generally viewed religion threefold -- as defined by Varro (116-117BC) who divided religion into: Mythical, Natural (Philosophical), and Civil. Augustine in his City of God 6.5 also gives a detailed account of this Roman threefold view of religion. I won't post it all here, however, since it's quite long.

The mythical aspect of Roman religion is what we commonly think of as being the entirety of Roman religion. That being deities such as those you mentioned in your question: Jupiter, Mars, Venus, Juno, Minerva, along with all various subdivisions of these gods by epithets like: Juno Lucina (Goddess of Childbirth), or Juno Sospita (Goddess of War/Protector of the Roman People), or Juno Moneta (Goddess of War/She Who Warns the Romans). But they could also be physical elements deified, as found in Horace's Ode 3.13 - "Ode to the Bandusian Spring." They could even be deified abstractions, such as Fortuna, Concordia, Spes (Hope), etc. Or even a genius, an amorphous spiritual/divine entity which people, things, and/or places might posses -- such as the genius of the Roman people, or a genius of a legion, or even the genius of a club or civic organisation.

With this mythical side of religion, however, we must resist the desire to pin the label of "scripture" to Roman myths. There is no fixed canon. Myth played a very limited role in the actual worship of gods. Myth is all at once: high culture, common culture, a series of foundation stories, learning stories about the gods and their origins, as well as just a series of allegories all rolled up into one.

That's the mythical side. The philosophical side is the likes of the Stoics, and the Epicureans, and the Academy. Here are the works of Marcus Aurelius with his Meditations, and Lucretius with his De Rerum Natura, and Cicero with his Academia. Here is the analysis of the nature of the gods and of reality. I could go on and on and on about this, but I'll stop it here and just say: there is a rich textual history concerning the dialogue between these three major branches (Epicurean, Stoic, Academy). If you want to do more reading on the subject, or if you'd like me to answer separate questions concerning the Natural side of Roman religion, then please don't hesitate to ask.

Now for the civil side. Here is where we find the founders of the Roman cult: figures such as Romulus and Numa, who -- if we're to believe Livy -- paved the path to the state cult and other such organisations. The primary premise of the state cult is that there are two aspects of Roman politics: Spiritual and Material. A consul can do everything he can, but in the end what he does is material, and the Gods also play a very real part in Rome's success. Politicians are leaders of the state cult, but there are also other branches and major religious authorities: such as the Priestly Colleges. The most well known of these Colleges is the College of the Pontiffs, ruled over by the Pontifex Maximus. Pontifices held an advisory function to state concerns. They would advise the senate on questions relating to the divine; they would oversee official religious acts like public sacrifices. We also had the Flamines, and the Vestal Virgins as well. Not to mention the Haruspices and Augurs, Fetiales (a sort of diplomatic corps, or priestly foreign service, if you will), and the Quindecemviri -- who were the only men able to consult the Sibylline books (under pain of death) in times of dire need.

It is in the civic religion that we see the rituals and worship of gods. Here are the blood sacrifices, the public vows, the honouring of obligations. In the end the primary function of Roman worship is to forge a contract. Here's another large difference between the Judeo-Christian faiths and Roman religion. Roman religion is steeped more in orthopraxy than in orthodoxy; it is correctness of conduct; it is a meeting of contract details. It is not a system of beliefs. But that does not make it any less of a religion.