“If Father, and Son, and Holy Ghost be no otherwise three than as notions, names, or nominal distinctions; then to worship these distinctly, or together, as such, is to commit most gross and horrible idolatry: For albeit we are commanded to fear that great and dreadful name, “The Lord our God;” yet to worship a Father, a Son, and Holy Spirit, in the Godhead, as three, as really three as one, is by this doctrine to imagine falsely of God, and so to break the second commandment: but to worship God under the consideration of Father, and Son, and Holy Ghost, and to believe them as really three as one when I worship, being the sum and substance of the doctrine of the scriptures of God, there is really substantially three in the eternal Godhead.
But to help thee a little in thy study on this deep,
1. Thou must take heed when thou readest, there is in the Godhead, Father and Son, &c. that thou do not imagine about them according to thine own carnal and foolish fancy; for no man can apprehend this doctrine but in the light of the word and Spirit of God: “No man knoweth the Son but the Father; neither knoweth any man the Father save the Son; and he to whom the Son will reveal him.” If, therefore, thou be destitute of the Spirit of God, thou canst not apprehend the truth of this mystery as it is in itself, but will either by thy darkness be driven to a denial thereof; or if thou own it, thou wilt (that thy acknowledgement notwithstanding) falsely imagine about it.
2. If thou feel thy thoughts begin to wrestle about this truth, and to struggle concerning this, one against another, take heed of admitting of such a question, How can this thing be? for here is no room for reason to make it out; here is only room to believe it is a truth. You find not one of the prophets propounding an argument to prove it, but asserting it; they let it lie, for faith to take it up, and embrace it.
“The grace of our Lord Jesus Christ, and the love of God, and the communion of the Holy Ghost, be with you all. Amen.””
~John Bunyan, The works of that eminent servant of Christ, Mr. John Bunyan, Volume 5, pp. 286-287 (link: The works of that eminent servant of Christ, Mr. John Bunyan – John Bunyan – Google Books)