The Old Roman Symbol was a short “creed” used in the baptism of Christians beginning in the 2nd or 3rd century. Muslims, Atheist and others, often claim that Christianity is divided since groups such as Jehovah Witnesses, Mormons, and other non-trinitarian groups, teach and believe things contrary to the majority of Christians.  However, these groups are not Christian, since one had to subscribe to the beliefs laid out in the creed which reads as follows:

“Old Roman Symbol”

I believe in God the Father almighty;
and in Christ Jesus His only Son, our Lord,
Who was born of the Holy Spirit and the Virgin Mary,
Who under Pontius Pilate was crucified and buried,
on the third day rose again from the dead,
ascended to heaven,
sits at the right hand of the Father,
whence He will come to judge the living and the dead;
and in the Holy Spirit,
the holy Church,
the remission of sins,
the resurrection of the flesh
(the life everlasting)

The Old Roman Symbol is one of the earliest expressions of Jesus’ divnity and was based on the rule of faith found in the Scriptures and in writings of several early church fathers:

Irenaeus:

Unity of the Faith of the Church Throughout the Whole World.
1. The Church, though dispersed through our the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith: [She believes] in one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations of God, and the advents, and the birth from a virgin, and the passion, and the resurrection from the dead, and the ascension into heaven in the flesh of the beloved Christ Jesus, our Lord, and His [future] manifestation from heaven in the glory of the Father “to gather all things in one,” and to raise up anew all flesh of the whole human race, in order that to Christ Jesus, our Lord, and God, and Saviour, and King, according to the will of the invisible Father, “every knee should bow, of things in heaven, and things in earth, and things under the earth, and that every tongue should confess” to Him, and that He should execute just judgment towards all; that He may send “spiritual wickednesses,” and the angels who transgressed and became apostates, together with the ungodly, and unrighteous, and wicked, and profane among men, into everlasting fire; but may, in the exercise of His grace, confer immortality on the righteous, and holy, and those who have kept His commandments, and have persevered in His love, some from the beginning [of their Christian course], and others from [the date of] their repentance, and may surround them with everlasting glory.

The Old Roman Symbol was used to “vet” who was and was not Christian. In fact, one would have to verbally subscibe to it in order to enter the Church. And all Christian Churches , or communities, had to pass down the beliefs expressed or they were not considered Christian:

2. As I have already observed, the Church, having received this preaching and this faith, although scattered throughout the whole world, yet, as if occupying but one house, carefully preserves it. She also believes these points [of doctrine] just as if she had but one soul, and one and the same heart, and she proclaims them, and teaches them, and hands them down, with perfect harmony, as if she possessed only one mouth. For, although the languages of the world are dissimilar, yet the import of the tradition is one and the same. For the Churches which have been planted in Germany do not believe or hand down anything different, nor do those in Spain, nor those in Gaul, nor those in the East, nor those in Egypt, nor those in Libya, nor those which have been established in the central regions of the world. But as the sun, that creature of God, is one and the same throughout the whole world, so also the preaching of the truth shineth everywhere, and enlightens all men that are willing to come to a knowledge of the truth. Nor will any one of the rulers in the Churches, however highly gifted he may be in point of eloquence, teach doctrines different from these (for no one is greater than the Master); nor, on the other hand, will he who is deficient in power of expression inflict injury on the tradition. For the faith being ever one and the same, neither does one who is able at great length to discourse regarding it, make any addition to it, nor does one, who can say but little diminish it. – Irenaeus,  Against Heresies ch 1. 9-10

Now, that we may not suffer ought of this kind, we must needs hold the rule of the faith without deviation, and do the commandments of God, believing in God and fearing Him as Lord and loving Him as Father. Now this doing is produced by faith: for Isaiah says: If ye believe not, neither shall ye understand. And faith is produced by the truth; for faith rests on things that truly are. For in things that are, as they are, we believe; and believing in things that are, as they ever are, we keep firm our confidence in them. Since then faith is the perpetuation of our salvation, we must needs bestow much pains on the maintenance thereof, in order that we may have a true comprehension of the things that are. Now faith occasions this for us; even as the Elders, the disciples of the Apostles,66 have handed down to us. First of all it bids us bear in mind that we have received baptism for the remission of sins, in the name of God the Father, and in the name of Jesus Christ, the Son of God, who was incarnate and died and rose again, and in the Holy Spirit of God. And that this baptism is the seal of eternal life, and is the new birth unto God, that we should no longer be the sons of mortal men, but of the eternal and perpetual God; and that what is everlasting and continuing is made God;67 and is over all things that are made, and all things are put under Him; and all the things that are put under Him are made His own; for God is not ruler and Lord over the things of another, but over His own; and all things are God’s; and therefore God is Almighty, and all things are of God.

For it is necessary that, things that are made should have the beginning of their making from some great cause; and the beginning of all things is God. For He Himself was not made by any, and by Him all things were made. And therefore it is right first of all to believe that there is One God, the Father, who made and fashioned all things, and made what was not that it should be, and who, containing all things, alone is uncontained.69 Now among all things is this world of ours,70 and in the world is man: so then this world also was formed by God.

Thus then there is shown forth One God, the Father, not made, invisible, creator of all things; above whom there is no other God, and after whom there is no other God. And, since God is rational, therefore by (the) Word He created the things that were made; and God is Spirit, and by (the) Spirit He adorned all things: as also the prophet says: By the word of the Lord were the heavens established, and by his spirit all their power. Since then the Word establishes, that is to say, gives body73 and grants the reality of being, and the Spirit gives order and form to the diversity of the powers; rightly and fittingly is the Word called the Son, and the Spirit the Wisdom of God.74 Well also does Paul His apostle say: One God, the Father, who is over all and through all and in its all. For over all is the Father; and through all is the Son, for through Him all things were made by the Father; and in us all is the Spirit, who cries Abba Father, and fashions man into the likeness of God.75 Now the Spirit shows forth the Word, and therefore the prophets announced the Son of God; and the Word utters the Spirit, and therefore is Himself the announcer of the prophets, and leads and draws man to the Father.

This then is the order of the rule of our faith, and the foundation of the building, and the stability of our conversation: God, the Father, not made, not material, invisible; one God, the creator of all things: this is the first point76 of our faith. The second point is: The Word of God, Son of God, Christ Jesus our Lord, who was manifested to the prophets according to the form of their prophesying and according to the method of the dispensation of the Father:77 through whom all things were made; who also at the end of the times, to complete and gather up78 all things, was made man among men, visible and tangible,79 in order to abolish death and show forth life and produce a community of union80 between God and man. And the third point is: The Holy Spirit, through whom the prophets prophesied, and the fathers learned the things of God, and the righteous were led forth into the way of righteousness; and who in the end of the times was poured out in a new way a upon mankind in all the earth, renewing man unto God.

And for this reason the baptism of our regeneration proceeds through these three points: God the Father bestowing on us regeneration through His Son by the Holy Spirit. For as many as carry (in them) the Spirit of God are led to the Word, that is to the Son; and the Son brings them to the Father; and the Father causes them to possess incorruption. Without the Spirit it is not possible to behold the Word of God, nor without the Son can any draw near to the Father for the knowledge of the Father is the Son,84 and the knowledge of the Son of God is through the Holy Spirit; and, according to the good pleasure of the Father, the Son ministers and dispenses the Spirit to whomsoever the Father wills and as He wills. – Ireaneus, The Demonstration of the Apostolic Preaching

Tertullian:

Tertullian (c. 155 – c. 220 AD) uses the phrases “rule of faith” and “rule of truth”:

Let our “seeking,” therefore be in that which is our own, and from those who are our own, and concerning that which is our own, – that, and only that, which can become an object of inquiry without impairing the rule of faith.

— Tertullian, The Prescription Against Heretics

Conclusion

Anyone who claims today to be Christian, but does not subscibe to the foundational doctrines as described in the Old Roman Symbol, is not a true Christian. Muslims and Atheist can claim that Jehovah Witnesses, Mormons, Seventh Day Adventist are Christians. However, since they do not subscribe to the basic truths of Christ and would be unable to confirm anything laid out in the Old Roman Symbol,  they are clearly not Christian.