This looks like an empty slogan: something that sounds appealingly simple and upon examination, has no useful meaning.
First, it is not itself "written in Gods Word". It is an assumption one can make by reading in many places that the Scripture is very important, a necessary part in Christian education, inspired by God, useful in disputes. Also in many places we are told that faith in Jesus is necessary for salvation, and we obviously get that faith in great part from the Scripture. But nowhere does the Scripture say it with that simplicity as you said. The Scripture does not say "If something is not recorded in Scripture, it is not necessary for you to know", or "All that you need to know is in the Scripture". Christ Himself surely made refernce to common knowledge of His time (e.g. the disaster of the Tower of Siloe, Lk 13:4) and quoted things that are not in the Christian Scripture (John 7:38; Jude quotes from Enoch, Jude 1:14).
Second, you worry of the "necessary" as if there is any harm in the unnecessary. The scripture does not tell you "Find out what is necessary and discard the rest". In fact, Chirst tauight the exactly opposite attitude. For example, in the Parable of the Unprofitable Servant (Luke 17:10) He taught quite the opposite, -- that we should not stop after we have done what is "commanded of us". The Church does not say that prayers to the saints are strictly necessary (at the Eucharistic Prayer, Mary and saints are mentioned but not prayed to; at the Penitential Rite their prayers are asked thusly "I ask Blessed Mary ever virgin and all the angels and saints, and you my brothers and sisters to pray for me to the Lord our God"; at the saints' holidays no special prayer is said to the saint being celebrated). So let us say, you decide to avoid the private devotions to saints on that score. What feeling is driving you if not spiritual sloth? Why are you not curious how Mary ended her days; how Sts Peter and Paul died; how the Trinity doctrine get established; what errors the Church fell into and how they were corrected?
Thirdly, while the scripture does not say that only the Scripture is necessary for salvation, it described Christ giving enormous powers to the Apostolic Church to "bind and loose", forgive sins, baptize and teach (Mt 16:19, Mt 18:18 Jn 20:23, Mk 17:15-16, Mt 28:19-20). In neither of these commissions Jesus says "write a book and list everything in it". Why do you run away from the authority that the Church has?
“The Scripture does not say “If something is not recorded in Scripture, it is not necessary for you to know”, or “All that you need to know is in the Scripture”. Christ Himself surely made refernce to common knowledge of His time (e.g. the disaster of the Tower of Siloe, Lk 13:4) and quoted things that are not in the Christian Scripture (John 7:38; Jude quotes from Enoch, Jude 1:14).”
I simply stated that as far as I can tell, there is nothing that is not recorded in the Holy Bible that a person needs for Salvation.
“Second, you worry of the “necessary” as if there is any harm in the unnecessary.”
Harm in the unnecesary? Not really, it is just unnecessary, unneeded. Not important in regards to Salvation.
“What feeling is driving you if not spiritual sloth?”
Love of my Lord and Saviour and fellow man.
“Why are you not curious how Mary ended her days”
Why would I be? The way Christ ended His on earth are what is important.
“how Sts Peter and Paul died”
I actually did a study many years ago on how the apostles met their ends. It was very interesting, but not necessary to know for Salvation.
“how the Trinity doctrine get established”
We actually have been taught this in Protestant churches.
“what errors the Church fell into and how they were corrected?”
Not being a Catholic, I don’t really concern myself with what errors that church has made and how they were corrected. I know the big ones. I know the current one.
“Thirdly, while the scripture does not say that only the Scripture is necessary for salvation”
No, scripture only tells us what or rather WHO is needed for Salvation.