There are a couple of ways of answering this question, the first based on Bible cross references:
gold - faith (1 Peter 1:7)
white raiment - pure character (Revelation 7:13-14)
eyesalve - spiritual discernment (John 9:1-11, John 16:7-11)
The other approach is to cite a couple of SOP quotes, that clearly define the symbols. See below.
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To be without the graces of the Spirit of God is sad indeed; but it is a more terrible condition to be thus destitute of spirituality and of Christ, and yet try to justify ourselves by telling those who are alarmed for us that we need not their fears and pity. Fearful is the power of self-deception on the human mind! What blindness! setting light for darkness and darkness for light! The True Witness counsels us to buy of Him gold tried in the fire, white raiment, and eyesalve. The gold here recommended as having been tried in the fire is faith and love. It makes the heart rich; for it has been purged until it is pure, and the more it is tested the more brilliant is its luster. The white raiment is purity of character, the righteousness of Christ imparted to the sinner. This is indeed a garment of heavenly texture, that can be bought only of Christ for a life of willing obedience. The eyesalve is that wisdom and grace which enables us to discern between the evil and the good, and to detect sin under any guise. God has given His church eyes which He requires them to anoint with wisdom, that they may see clearly; but many would put out the eyes of the church if they could; for they would not have their deeds come to the light, lest they should be reproved. The divine eyesalve will impart clearness to the understanding. Christ is the depositary of all graces. He says: "Buy of Me." {4T 88.2}
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In my last vision I was shown that even this decided message of the True Witness had not accomplished the design of God. The people slumber on in their sins. They continue to declare themselves rich and having need of nothing. Many inquire: Why are all these reproofs given? Why do the Testimonies continually charge us with backsliding and with grievous sins? We love the truth; we are prospering; we are in no need of these testimonies of warning and reproof. But let these murmurers see their hearts and compare their lives with the practical teachings of the Bible, let them humble their souls before God, let the grace of God illuminate the darkness, and the scales will fall from their eyes, and they will realize their true spiritual poverty and wretchedness. They will feel the necessity of buying gold, which is pure faith and love; white raiment, which is a spotless character made pure in the blood of their dear Redeemer; and eyesalve, which is the grace of God and which will give clear discernment of spiritual things and detect sin. These attainments are more precious than the gold of Ophir. {3T 254.2}
the eyesalve is spiritual discernment given by Grace of God
anything else?
how do we obtain these things that are needed in these last days??
This question is hard to answer in a few words. But it's really important, and I just wrote an essay titled "Washing Our Robes", and so I'll take a shot at providing a brief summary of how we obtain the things that Revelation 3:18 mentions.
1. The Bible says that we are covered by Christ's robe, or by a robe of self-righteousness (Isaiah 61:10, Isaiah 64:6).
2. Many people like the self-righteous robe, and wish to keep it (Daniel 12:10, Matthew 22:11-14, Revelation 22:11).
3. If we instead want Christ's robe, what then? The Bible mentions several core steps, including exercising the power of choice to follow the Lord, submitting our wills, and confessing our sins (Joshua 24:15, James 4:7, 1 John 1:9).
4. These are some of the elements of heart religion, and wearing Christ's robe is impossible without such religion. In other words, wearing Christ's robe on an external basis doesn't make sense (Psalm 40:6-8, Mark 12:33).
5. If we do embrace heart religion, then the Bible says in a variety of ways that we have Christ living within us (John 6:54-57, Galatians 2:20). We "put on Christ" (Galatians 3:27).
6. If we have Christ living within us, then various attributes of the Spirit are imparted to us, and our characters are transformed over time (John 16:7-11, 1 Thessalonians 5:23).
7. Part of this transformation occurs as we emulate Christ's life during the Incarnation, for example His devotion to service (Matthew 4:23), and His deep investment in prayer (Luke 6:12).
8. Another aspect of this is Christ's promise to help us in times of great temptation and trial, by sharing the victory that He worked out in human nature during His time on earth (Hebrews 2:14-18, Hebrews 4:14-16).
9. All of this fits under the Revelation 7:14 statement about washing our robes of character in the blood of the Lamb.
10. The Bible says that following the Lamb eventually becomes a natural thing to do (Revelation 14:4).
