Hebrews 5:13 For every one that useth milk is unskilful in the word of righteousness: for he is a babe.
Hebrews 5:14 But strong meat belongeth to them that are of full age, even those who by reason of use have their senses exercised to discern both good and evil.
You have a very good point, many Christians in the world and even here on forums will say "Just faith, saved by Grace". But that is counter to everything the Word of God says, "2 Timothy 2:15 Study to shew thyself approved unto God, a workman that needeth not to be ashamed, rightly dividing the word of truth." We are told plainly we need to study the Word of God to be "approved unto God", so that we are not "ashamed". Which ties in the ten virgins, the five fools are Christians thinking "all I need to do is believe", but then Jesus rejects them because they have no oil and do not shine the Glory of God with their actions.
When we are given the free gift of faith we are babes, we know the "milk" or basics, that there is a God and his Son Jesus Christ died for our sins. But if that is the extent of your knowledge of God and Christianity, only to fall back to "saved by grace", you leave yourself open to satan's attacks having no foundation of knowledge or the understanding to "discern both good and evil". You could be sinning and being vile people, you would not know, unless you study the Word of God and grow into understand the "meat".