The problem with Christless religion is that you never know if you have done enough to satisfy the divine standard.  Paul's letter to the church in Rome makes it much more straightforward: everybody has sinned and nobody has reached His standard (Romans 3:23); but (for the believer) the death of Jesus has satisfied all of the Father's demands and given us a righteousness we could never have earned (Romans 4:25).

That radically changes our relationship with God.  In regard to our past sins, we are justified; in regard to our future punishment, we are saved; and in regard to our present relationship, we are reconciled.  So, we can be assured of total security before God: He has transformed our past, present and future so that we can be freed from guilt to receive His love.  That transformed relationship should bring us great joy.  God is for us and has nothing against us (Romans 8:31-32).

So why do some Christians lack joy?  They are looking to themselves and not to Christ.  Although they may know that the sacrifice of Jesus Christ has dealt with the past, secured the present and the future, they have not accepted the truth for themselves.  They may be guiltily hiding from God, nervous to presume on a relationship with Him, even though He has reconciled them to Himself.

Away with such self-defeating suggestions; they are devilish lies (John 8:44)!  The Lord Jesus is alive, to relate with us and intercede for us (Hebrews 7:25).  So rejoice!  And as we do, our friends and colleagues will notice the difference and ask us why.  Then we can tell them of the glorious gospel which brings total security and leads to joy.