In 1 Peter 4:18, what does Peter mean by the righteous having difficulty to be saved?

In 1 Peter 4:18, the Apostle Peter is talking about salvation.

Salvation simply means "rescue".

Peter is talking about rescue from sufferings on earth not from damnation in hell as the immediate context shows.

1 Peter 4:12-19 (NASB)

12 Beloved, do not be surprised at the fiery ordeal among you, which comes upon you for your testing, as though some strange thing were happening to you; 13 but to the degree that you share the sufferings of Christ, keep on rejoicing, so that also at the revelation of His glory you may rejoice with exultation. 14 If you are reviled for the name of Christ, you are blessed, because the Spirit of glory and of God rests on you. 15 Make sure that none of you suffers as a murderer, or thief, or evildoer, or a troublesome meddler; 16 but if anyone suffers as a Christian, he is not to be ashamed, but is to glorify God in this name. 17 For it is time for judgment to begin with the household of God; and if it begins with us first, what will be the outcome for those who do not obey the gospel of God? 18 And if it is with difficulty that the righteous is saved, what will become of the godless man and the sinner? 19 Therefore, those also who suffer according to the will of God shall entrust their souls to a faithful Creator in doing what is right.

The reason why the righteous has difficulty with having salvation from sufferings on earth is because he still has the ability to sin ( John 1:5-10). The point is that it is not impossible for the rigtheous to be saved from sufferings on earth but rather, it's possible yet with difficulty.

Who is going to harm you if you are eager to do good? But even if you should suffer for what is right, you are blessed. 1 Peter 3:13-14 (NASB)

For it is better, if it is God’s will, to suffer for doing good than for doing evil.For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. 1 Peter 3:17-18 (NASB)

A widespread belief around the middle of the first century was that the parousia would happen in the lifetimes of those still alive.

For example, as Bart D. Ehrman points out in Forged, p106, Paul expected the second coming of Jesus imminently. This is made particularly clear in 1 Thessalonians 4:17, where he expected to be one of those present at the end: "Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord." In 1 Corinthians chapter 7, Paul is telling his followers that it is better not to make new arrangements such as marriage when time is so short, but that if a person must marry, then so be it.

Rhoads, Dewey and Michie, in Mark as Story, page 80, explain that the author of Mark's Gospel believed that the rule of God would be established here on earth when Jesus returned on clouds of glory during the lifetimes of the people to whom Jesus had preached (Mark 9:1: "And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power", Mark chapter 13).

1 Peter 1:7 contains a suggestion of the imminent return of Jesus, as does 1 Peter 4:7:

1 Peter 1:7: That the trial of your faith, being much more precious than of gold that perisheth, though it be tried with fire, might be found unto praise and honour and glory at the appearing of Jesus Christ:

1 Peter 4:7: But the end of all things is at hand: be ye therefore sober, and watch unto prayer.

In chapter 4, the author of First Peter is talking about the imminent return of Jesus, so we should place his concept of salvation in that context. 1 Peter 4:17,18 goes on:

17: For the time is come that judgment must begin at the house of God: and if it first begin at us, what shall the end be of them that obey not the gospel of God? 18: And if the righteous scarcely be saved, where shall the ungodly and the sinner appear?

Our author says that the righteous will be judged first ("begin at us"), and that their salvation (in the life to come) is not assured. This is a literal quotation, word for word, of the Septuagint (LXX) version of Proverbs 11:31. This is not original to 1 Peter, so the author did not consciously choose the words, and therefore we can not be too literal in reading meaning into his use of 'righteous' or 'saved'. However, I believe that it is intended here as a warning against sin, for those who want to be saved.

I would not separate this 'salvation' from salvation from hell, for I do not think there is any other kind of salvation. Sin, hell, the world and the devil are all arrayed against the believer making our death to the world and our life in Christ a target of conflict; an inwardly felt weakness and struggle. This target (the faith of believers) is the end direction of all the energy, malice and aggression that the Devil and all the whole world enslaved to sin aims at, making our salvation very difficult. Not that we can ever loose it but that it is a road of conflict by nature of the evil in this world and the remans of sin within our own selves.

On the other hand, sinners often have an easy life for their lusts are harmonious with the desires of the whole world. If a sinner has wealth, or very good looks or a very reputable name in business or religion, he may live an easy life without intense inner conflicts against sin, for he freely follows his lusts without conflict in the world and is praised for it by the world.

The statement is saying that if a real believer who 'feels so weak' and is often actually 'externally weak' in terms of worldly power, and who 'struggles daily' like a 'bruised reed and a smoldering wick', if they are saved through this kind of 'difficulty', how will the wicked find the final judgment to be after their life of ease and lack of struggle?

The answer is clear, sinners who have no struggle with a world of sin will find hell swallowing up their souls in sudden terror, whereas the struggling weak and tortured believer, whose faith seems always tested from without and within due to our backsliding hearts, will enter at last into glory forevermore. What a contrast! This is the point of it, a contrast of life in this world and one's corresponding destiny hereafter.