I Corinthians 15  Paul uses the word "vain" 5 times in this chapter:

v2 "...unless you believed in vain"
v10 " His grace toward me did not prove vain."
v14 " and If Christ has not been raised then our preaching is vain, your faith also is vain." 
v58  "... knowing that your toil is not in vain in the Lord." 

Vain - empty.. without worth.  Without faith in the whole gospel of Jesus Christ.. including the truth that He was raised from the dead...there is no worth in a christian religious life..

Paul made known "the gospel" to these "brethren".  He preached and they received it.  They needed to stand firm and "hold fast" to the Word.  "For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures, and that He was buried, and that He was raised on the third day according to the Scriptures..." 

The issue that Paul is dealing with is that some were arguing the there is no such thing as the "resurrection of the dead".   Paul teaches that a gospel without the resurrection is no gospel at all!  It is vain - worthless - empty.  Our faith would be worthless - for if Christ was not resurrected our sins would not be forgiven.  If Christ was not raised from the dead..there is no use in preaching about Him.  If Christ did not rise from the grave then we have no hope.

But, Paul continues, the Truth is.. Christ was raised from the dead.  And because He did.. we "all shall be made alive" with Him!  Because He rose from the dead we have hope.. and it is a glorious hope that will be accomplished in all its fulness when He comes again.

Father in heaven,  we do not want to believe anything less than the whole gospel of Jesus Christ.  We put our faith in the truth of Your Word and want to live fully in obedience, by your grace.  Fill us Sprit of Truth with deeper faith and divine understanding.. that we might be "steadfast, immovable, always abounding in the work of the Lord.. " We ask this in the name of the Lord Jesus Christ. Amen.