26 And he said, “The kingdom of God is as if a man should scatter seed
on the ground. 27 He sleeps and rises night and day, and the seed
sprouts and grows; he knows not how. 28 The earth produces by itself,
first the blade, then the ear, then the full grain in the ear. 29 But
when the grain is ripe, at once he puts in the sickle, because the
harvest has come.”
I have clear recollections of the time when I was a new believer. I was asked by my Bible study leader to memorize this passage in Galatians 2:
20 I have been crucified with Christ. It is no longer I who live, but
Christ who lives in me. And the life I now live in the flesh I live by
faith in the Son of God, who loved me and gave himself for me. 21 I do
not nullify the grace of God, for if righteousness were through the
law, then Christ died for no purpose.
This verse is about receiving a new identity in Christ, of dying to the old self. I had no clue at the time what it meant. One month later, I was freed from the fear of death and the fear of living a worthless life of no consequence. I attribute that change in me to those verses. The Holy Spirit made words I did not understand grow inside me, though I did not know how, just like the farmer in Jesus' parable.
I assume that assent to the Doctrine of the Trinity is the same. The Holy Spirit can lead an ignorant but faithful Christian into all truth, though it may take time. Understanding can produce faith, but the walk of faith can also produce understanding. Sometimes the Word must change us before we can understand what it means.
In the end, it is not assent to the doctrine of the Trinity that saves you. What saves you is embracing each member of the Trinity as they visit you. The doctrine is the light by which you can understand each visit. I remember crying at the thought that Jesus died on the cross for me personally. That was when I knew that Jesus was a real person, because I could not cry for an abstraction. Likewise, when I visited a Christian woman on her deathbed out of obedience, the Holy Spirit overpowered me and delivered me from a decade of depression. I knew that it was the Holy Spirit because I had been taught that there was a Holy Spirit and Joy one of its fruits. Finally, when in a dream I was warned of years of suffering about to begin and later struck deaf in one ear, then healed after reading a passage in Exodus about how the Father can cause deafness and sternly directed Moses to go to Egypt, I realized I had met with the Father.
These are not my only encounters with each member of the Trinity, but they are the most potent. The doctrines prepare you to recognize God however He chooses to appear to you. It is up to you to embrace God, whether as Father, Son, or Holy Spirit, or all three at once in an undifferentiated unity. It is that embrace, obedience, trust, and acceptance of God when He appears as He chooses to make Himself known that counts. That is what saves.
When Joshua met the man with the drawn sword in Joshua 5, why did he accept the man's declaration that he was the commander of the host of the Lord? It is because of a worship song in Deuteronomy 32 that Moses spoke to the people. Moses said that God would draw his sword and fight for his people, but they must take the words of the song to heart. That passage makes a big deal out of the fact that Joshua was present for the singing of that song. Joshua remembered the song and therefore recognized God on the day He came to visit him. That is what the doctrine of the Trinity does for us: it helps us recognize God when he visits us, so that we may not be deceived by counterfeits.