There are two questions about the passage in 1 Peter 4:17, 18.
Household/Family of God (v17)
This phrase "household of God" refers to the Christian community, see Eph 2:19, 1 Tim 3:15, Titus 1:7, Heb 3:2, 5, 6, 10:21, etc. This is conformed in the same verse where Peter writes, " … judgment to begin at the household of God; and if it begins with
us … ". Apparently, Ellicott agrees:
Begin at the house of God.--The phrase contains an obvious reference
to Ezekiel 9:6 (comp. also Jeremiah 25:29). Who are meant by the
"house of God" is clear, not only from such passages as 1Peter 2:5;
1Corinthians 3:16; 2Thessalonians 2:4, but also from the immediate
addition, "and if first at us."
Further, Paul also uses the Temple as a metaphor of the Christian community, 1 Cor 3;16, 17. In this and the other famous metaphor of the Christian community - the wheat and weeds parable, the righteous and the unrighteous in the church co-exist and are separated at the judgement.
Difficult Salvation (v18)
God is a great saviour and thus I do not believe the difficulty of salvation refers to any difficulty with God but with us. There are several references to this idea such as 1 Cor 3:15, Job 19:20, and of course the verse that Peter quotes is from Prov 11:31. As the Pulpit commentary observes:
The righteous shall be requited in the earth, that is, chastised for
his transgressions. So it would be now, St. Peter says; judgment must
begin at the house of God. He adopts the inexact Septuagint
translation for its substantial truth, as we now sometimes use
versions which are sufficient for practical purposes, though we know
them to be critically inaccurate. We observe again the absence of
marks of quotation, as often in St. Peter. Bengel well remarks that
the awful "scarcely" (μόλις σώζεται) is softened by 2 Peter 1:11.
Ellicott reaches a similar conclusion:
The fact that they are "scarcely" saved "imports not," according to
Leighton, "any uncertainty or hazard in the thing itself to the end,
in respect of the purpose and performance of God, but only the great
difficulties and hard encounters in the way." This is only partly
true. The Apostle is rather thinking of the final judgment than of the
life of trial; and he means that there was but little margin left: a
very few more falls, a few more refusals to follow the calls of grace,
and they would have been lost. Doubtless, when the best of us looks
back, in the light of the last day, upon all that he has been through,
he will be amazed that he ever could be saved at all. Yet Bengel well
calls us to see the other side of the picture in 2Peter 1:11.