If we seek ultimate joy in other things, and our Christianity is dry and lifeless, it only shows that we really don't believe He is supremely and sufficiently enjoyable and the portion of our soul forever.
If we live under a constant burden of guilt and trying to be good, and if we are constantly trying to justify ourselves by things we do, build ourselves up before God and others, it only shows that we really don't believe the gospel. We really don't believe it is all of grace and none of merit. We really don't believe Christ is our complete righteousness, our identity, our Rock, our foundation.
If we do not seek the lost, do not love and endeavor to love those around us, do not them about Jesus, it shows that we do not truly believe our Christianity. We do not believe they are creations, like us, sinners, like us, lost, as we were. We do not truly believe in a joyful heaven, in the glory of Jesus Christ, and we do not truly believe in the horrors of hell. We do not truly believe in salvation all of grace, by Christ alone, which we have done nothing to deserve, only to spurn.
We can talk about this doctrine and that. We can profess to believe them. We can prove them on paper. But our actions betray what we truly believe, what we truly apprehend in our souls and hold fast to.
“An infidel once met a Christian and said, ‘I know you do not believe your religion.’ ‘Why?’ asked the Christian. ‘Because,’ said the other, ‘for years you have passed me on my way to my house of business. You believe, do you not, that there is a hell, into which men’s spirits are cast?’ ‘Yes, I do,’ said the Christian. ‘And you believe that unless I believe in Christ I must be sent there?’ ‘Yes.’ ‘You do not, I am sure, because if you did, you must be a most inhuman wretch to pass by, day by day, and never tell me about it or warn me of it.’” (Charles Spurgeon)