We understand the scripture by the Holy Spirit, weather that is by direct revelation from Him as we hear or read the scripture; or also indirectly from another believer as he explains the scripture to us as has been reveled to him by the Holy Spirit.
Now with regards to preaching, the word of God has expressly said; “So faith comes from hearing, and hearing by the word of Christ.” (Rom.10.17.NASB) We are all saved one way; by the grace of God through faith in Jesus alone. The scripture says; “Whoever will call on the name of the Lord will be saved.” (Rom.10.13.NASB) Jesus is His name, and we know about Him through scripture alone. “And there is salvation in no one else; for there is no other name under heaven that has been given among men by which we must be saved.” (Acts.4.12.NASB)
Therefore men are saved by the grace of God through faith in Jesus alone, and faith comes from hearing about Him through the scripture alone, and the understanding of which is given by the Holy Spirit. Therefore from now on, all my preaching will be expository from the scripture alone. I will open up the scripture, proclaim and explain the scripture alone.
C. H. Spurgeon said; “Faith has three parts; knowledge, belief, and trust.” His knowledge of this must have come through this passage of scripture. “How then will they call on Him (Trust in Him) in whom they have not believed? How will they believe in Him whom they have not heard? And how will they hear without a preacher?” (Rom.10.14.NASB) Many unregenerate men will not pick up the scripture and read it themselves; and it is through the scripture alone that they will come to know Him. So let us publicly proclaim and explain the scripture alone; so that men will hear about Jesus and come to know Him, believe in Him, and trust in Him alone.
A Slave of Jesus Christ
(The words in italics and in parentheses within the scripture quotation are my understanding of what is stated.)