“There are three that bear record in Heaven, the Father, the Word, and the Holy Ghost, and these three are One” (1 John 5:7).
God is but one, yet there are three distinct persons subsisting in one Godhead.  This is a sacred mystery, which the light within man could never have discovered.  As the two natures in Christ (God-man), yet but one person, is a wonder; so are the three persons, yet but one Godhead.  Here is a deep truth, the Father God, the Son God, the Holy Ghost God; yet not three Gods, but one God.  The three persons in the blessed Trinity are distinguished, but not divided; three substances, but one essence. 
  This is a divine riddle, where one makes three, and three make one.  Our narrow thoughts can no more comprehend the Trinity in Unity, than a nut-shell will hold all the water in the sea.  Let me shadow it out by a similitude.  In the body of the sun, there are the substance of the sun, the beams, and the heat.  The beams are begotten of the sun, the heat proceeds both from the sun and the beams; but these three, though different, are not divided.  They all three make but one sun: so in the blessed Trinity, the Son is begotten of the Father, the Holy Ghost proceeds from both; yet though they are three distinct persons, they are but one God.  First, let me speak of the Unity in Trinity; then of the Trinity in Unity.
  I. OF THE UNITY IN TRINITY.  The Unity of the persons in the Godhead consists of two things.
The identity of essence.  In the Trinity there is a oneness in essence.  The three persons are of the same divine nature and substance; so that in «there are no degrees in the Godhead»; one person is not God more than another.
  The Unity of the Persons in the Godhead consists in the mutual inbeing of them, or their being in one together.  The three persons are so united that one person is in another, and with another.  “Thou, Father, art in me, and I in thee” (John 17:21).
  II.  LET ME SPEAK OF THE TRINITY IN UNITY.
The first Person in the Trinity is God the Father.  He is called the first person, in respect of order, not dignity: for God the Father has no essential perfection which the other persons have not; he is not more wise, more holy, more powerful than the other persons are.  There is a priority, not a superiority.
  The second Person in the Trinity is Jesus Christ, who is begotten of the Father before all time.  “I was set up from everlasting, from the beginning, or ever the earth was.  When there were no depths I was brought forth; when there were no fountains abounding with water.  Before the mountains were settled, before the hills, was I brought forth” (Proverbs 8:23-25).  This Scripture declares the eternal generation of the Son of God.  This second person in the Trinity, who is Jehovah, is become our Jesus.  The Scripture calls him the branch of David, (Jer. 23:5), and I may call him the flower of our nature.  “By him all that believe are justified” (Acts 13:39).
  The third Person in the Trinity is the Holy Ghost, who proceeds from the Father and the Son, whose work is to illuminate the mind, and enkindle sacred motions.  The essence of the Spirit is in Heaven, and everywhere; but the influence of Him is in the hearts of believers.  This is that blessed Spirit who gives us the holy unction (I John 2:20).  Though Christ merits grace for us, it is the Holy Ghost that works it in us.  Though Christ makes the purchase, it is the Holy Ghost that makes the assurance, and seals us to the day of redemption.  Thus I have spoken of all the three persons. The Trinity of persons may be proved out of Matt 3:16.  “Jesus, when He was baptized, went up straightway out of the water, and he saw the Spirit of God descending like a dove, and lighting upon him; and lo, a voice from heaven, saying, This is my beloved Son”.  Here are three names given to the three persons.  He who spake with a voice from Heaven was God the Father; He who was baptized in Jordan was God the Son; He who descended in the likeness of a dove was God the Holy Ghost.  Thus I have shown you the Unity of essence, and the Trinity of persons.
  For confutation:  This confutes the Jews and Muslims, who believe only the first person in the Godhead.  Take away the distinction of the persons in the Trinity, and you overthrow man’s redemption; for God the Father being offended with man for sin, how shall he be pacified without a mediator?  This mediator is Christ, who makes our peace.  Christ having died, and shed his blood, how shall this blood be applied but by the Holy Ghost?  Therefore, if there be not three persons in the Godhead, man’s salvation cannot be wrought out; if there be no second person in the Trinity, there is no redeemer; if no third person, there is no comforter.  Thus the plank is taken away by which we get to Heaven.
  For exhortation:  Believe this doctrine of the Trinity of persons in the unity of essence.  The Trinity is purely an object of faith; the plumbline of reason is too short to fathom this mystery; but where reason cannot wade, there faith may swim.  There are some truths in religion that may be demonstrated by reason, as that there is a God: but the Trinity of persons in the Unity of essence is wholly supernatural, and must be believed by faith. This sacred doctrine is not against reason, but above it.  Those illuminated philosophers, that could find out the causes of things, and discourse of the magnitude and influence of the stars, the nature of minerals, could never, by their deepest search, find out the mystery of the Trinity.  This is of divine revelation, and must be adored with humble believing.  We can be no good Christians, without the firm belief of the Trinity.  How can we pray to God the Father but in the name of Christ, and through the help of the Spirit? How can we believe the glorious Trinity?
  If there be one God subsisting in three persons, then let us give equal reverence to all the persons in the Trinity.  There is not more or less in the Trinity; the Father is not more God than the Son and Holy Ghost.  There is an order in the Godhead, but no degrees; one person has not a majority or supereminence above another, therefore we must give equal worship to all the persons.  “That all men should honour the Son even as they honour the Father” (John 5:23).  Adore Unity in Trinity.
Obey all the persons in the blessed Trinity; for all of them are God!  Pray to the Father, in the name of the Son and receive the Holy Spirit..  “For whosoever shall call upon the name of the Lord shall be saved” (Romans 10:14).  Call on Jesus today, right now!
                                                                                                                                                        Thomas Watson