2Ti 4:4 And they shall turn away their ears from the truth, and shall be turned unto fables.
Tit 1:9 Holding fast the faithful word as he hath been taught, that he may be able by sound doctrine both to exhort and to convince the gainsayers.
Tit 2:1 But speak thou the things which become sound doctrine:
It has a two fold benefit for us, as it not only gives us this wonderful insight, it re-affirms in 675 AD the Apostles Creed of 140 AD, Nicene Creed of 325 AD, and the Athanasian Creed of 500 AD.
I hope by looking at this Church Council we can obtain a better understanding of the Scriptures that are listed above in these post. In 2 Timothy the Apostle Paul is speaking of sound doctrine, he is doing it again in Titus. The understanding of the Trinity is sound doctrine, you can follow it as the Church moves through history. It is not a new doctrine, but it is part of what Jude speaks of in Jude 1:3 saying, "Beloved, when I gave all diligence to write unto you of the common salvation, it was needful for me to write unto you, and exhort you that ye should earnestly contend for the faith which was once delivered unto the saints." There is a faith that was once delivered to the saints, this faith has crossed the expanse of time and reaches to us in its fullness and glory.
This is important for us to examine, for this is the faith, the Christian faith. I have been exposed to teaching during my life that would contradict this understanding of the Trinity. For a time I was believing an incorrect view of God because of error introduced into what was supposed to be the Christian faith. It went so far as to convince me also to be re-baptized in a non-Trinitarian way. These other doctrines are dangerous and should be avoided. I was taken captive by them because I had not been educated in sound doctrine and I was vulnerable to false teaching. There are Churches among us that call themselves Christian, but hold an entirely different view of God. These Churches are not Christian Churches as defined by sound doctrine. I know it is uncomfortable for us to exclude wonderful friends and family who may be a part of such a Church. But we are commanded by Scripture to hold to and proclaim sound doctrine. We cannot just sit quietly by letting damaging doctrines go unchallenged just because we might offend family or friends.
With that being said, let's look at this wonderful statement of faith from the Church in 675 AD. I will not post its entirety, just what is relevant to our topic here. I would recommend doing a search on the Eleventh Council of Toledo and read it in its entirety.
(311  This is the way of speaking about the Holy Trinity as it has been handed down: one must not call it or believe it to be threefold, but Trinity. Nor can it properly be said that in the one God there is the Trinity, but the one God is the Trinity. In the relative names of the persons the Father is related to the Son, the Son to the Father, and the Holy Spirit to both. While they are called three persons in view of their relations, we believe in one nature or substance. Although we profess three persons, we do not profess three substances, but one substance and three persons. For the Father is Father not with respect to Himself but to the Son, and the Son is Son not to Himself but in relation to the Father; and likewise the Holy Spirit is not referred to Himself but is related to the Father and the Son, inasmuch as He is called the Spirit of the Father and the Son. So when we say 'God', this does not express a relationship to another, as of the Father to the Son or of the Son to the Father or of the Holy Spirit to the Father and the Son, but 'God' refers to Himself only.
312  For, if we are asked about the single persons, we must confess that each is God. Therefore, we say that the Father is God, the Son is God, the Holy Spirit is God ' each one distinctly; yet there are not three gods, but one God. Similarly, we say that the Father is almighty, the Son is almighty, the Holy Spirit is almighty, each one distinctly; yet there are not three almighty ones, but one Almighty, as we profess one light and one principle. Hence we confess and believe that each person distinctly is fully God, and the three persons together are one God. Theirs is an undivided and equal Godhead, majesty and power, which is neither diminished in the single persons nor increased in the three. For it is not less when each person is called God separately, nor is it greater when all three persons are called one God.
313  This Holy Trinity, which is the one true God, is not without number; yet it is not comprised by number, because in the relationships of the persons there appears number, but in the substance of the Godhead nothing is comprised that could be counted. Therefore they imply number only in so far as they are mutually related, but they lack number in so far as they are by themselves (ad se). For this Holy Trinity has so much one name referring to its nature that it cannot be used in the plural with relation to the three persons. This then is, in our faith, the meaning of the saying in Holy Scripture: "Great is our Lord, abundant in power, and of His wisdom there is no number" (Ps. 147 (146) 5 Vulg.).
(The Trinity in the oneness)
314  However, though we have said that these three persons are one God, we are not allowed to say that the same one is the Father who is the Son, or that He is the Son who is the Father, or that He who is the Holy Spirit is either the Father or the Son. For He is not the Father who is the Son, nor is the Son He who is the Father, nor is the Holy Spirit He who is the Father or the Son, even though the Father is that which the Son is, the Son that which the Father is, the Father and the Son that which the Holy Spirit is, that is one God by nature. For, when we say: He who is the Father is not the Son, we refer to the distinction of persons; but when we say: the Father is that which the Son is, the Son that which the Father is, and the Holy Spirit that which the Father is and the Son is, this clearly refers to the nature or substance, whereby God exists since in substance they are one; for we distinguish the persons, but we do not divide the Godhead.
 Hence, we recognise the Trinity in the distinction of persons and we profess the unity on account of the nature or substance. Thus, the three are one by nature, not as person.
(The undivided Trinity)
315  Nevertheless these three persons are not to be considered separable since, according to our belief, none of them ever existed or acted before another, after another, without another. For they are inseparable both in what they are and in what they do, because, according to our faith, between the Father who generates and the Son who is generated or the Holy Spirit who proceeds, there has not been an interval of time in which the one who generates would precede the one who is generated, or there would be no begotten one to Him who begets, or the Holy Spirit in His proceeding would appear later than Father or Son. For this reason we profess and believe that this Trinity is inseparable and distinct (inconfusa). We say, therefore, of these three persons, as our forefathers defined it, that they should be acknowledged, not separated. For if we listen to what Holy Scripture says about Wisdom: "She is a reflection of eternal light" (Wis. 7.26), we see that, as the reflection belongs inseparably to the light, so too, according to our confession, the Son cannot be separated from the Father. Therefore, neither do we confuse these three persons whose nature is one and inseparable, nor do we preach that they are in any way separable.
316  The Holy Trinity itself has indeed deigned clearly to reveal it to us: in these names by which He wanted the single persons to be known, it is impossible to understand one person without the other; one cannot conceive of the Father without the Son, nor can the Son be found without the Father. Indeed, the very relationship expressed in the personal names forbids us to separate the persons, for, though it does not name them together, it implies them. No one can hear any one of these names without necessarily understanding also the other. While then these Three are One and this One Three, each of the persons retains His own characteristics: The Father has eternity without birth; the Son has eternity with birth; the Holy Spirit has procession without birth with eternity.
May the Grace of God be upon each of you,