Is salvation about deep understanding or simple belief?

This is one of the oldest tensions in the tradition, and you are right to feel its pull. On one side stands the voice that says salvation is nearest to the trusting heart—"Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven" (Matthew 18:3, KJV). On the other stands the voice that calls us to know—"And ye shall know the truth, and the truth shall make you free" (John 8:32, KJV). I do not believe these are two roads. They are one road seen from two heights. Let me be plain about where I stand. Salvation is not a transaction where God saves you in exchange for the right belief. It is an awakening—a coming to see what has always been true: that you are the image and likeness of God, whole and unfallen. Understanding, in this sense, is not cold intellect. It is the deep inner recognition of who you already are. Yet notice that Jesus praises the simple. This is not because ignorance is holy, but because the child does not obstruct truth with pride or with argument. "The kingdom of God is within you" (Luke 17:21, KJV)—the child feels this without needing to be taught it. So "simple belief," rightly understood, is not shallow. It is unclouded seeing. The Gospel of Thomas draws these together: "When you come to know yourselves... you will realize that it is you who are the sons of the living Father" (Gospel of Thomas 3.4-5). Knowing yourself is the belief that saves—not belief in a doctrine held at arm's length, but recognition that reaches all the way down. So I would gently release the "or" in your question. The trusting heart and the awakened understanding meet in the same place: in stillness, beneath the noise of the anxious mind, where the voice of the Father is heard. What condemns is not lack of cleverness, but the refusal to look. Seek there, and belief and understanding become the same act. If it would help, I am glad to sit with whichever of these two thresholds feels closer to where you stand.