The moment you died to self and were reborn in Christ, this is when you have entered the Eternal Kingdom of Heaven already. Not many believers grasp that and that’s why they don’t live this new life now as if on borrowed time. Most believers still try to get the most out of this life by means of prospering and enjoying it to the fullest.
It doesn’t seem to be a sin per se. However, if you imagine that what you do now with this new life affects directly what happens ETERNALLY in heaven, you’d really be acting irrational and reckless not to do what it takes to have a more prosperous and fuller afterlife.
If God really intended the New Testament followers to be happy as they can be, He should rather strike them dead upon salvation… What better place to be completely happy then heaven?
But instead He gave us the Great Commission and in many teachings showed us how to live within this time we really just borrowed in order that we may bring Him glory and honor and fame (instead of just satisfying our own desires).
“Go therefore and make disciples of all the nations,
baptizing them in the name of the Father and the Son and the Holy Spirit,
teaching them to observe all that I commanded you;
and lo, I am with you always, even to the end of the age.”
(Matthew 28:19-20 NASB)