“He that saith, I know him, and keepeth not his commandments, is a liar, and the truth is not in him” (1 John 2:4)

Anyone that has the true knowledge of God would never willingly give himself to sin.
It's those who don’t know him that join the team of the people that use the phrase “Christianity is in the heart” and easily give themselves to sin.

“He that saith he abideth in him ought himself also so to walk, even as he walked.” (1 John 2:6).
Your deeds ought to correspond with God’s commandments. People call you Christian because of your behavior and appearance. You don’t need to tell people you are a Christian before they identify you as one.

“And hereby we do know that we know him, if we keep his commandments.” (1 John 2:3), the yardstick that you are a Christian is keeping God’s commandment.
You cannot keep God’s commandment in your heart and your actions and appearance exhibits opposite.

Christianity is in your action not something you claim to be somewhere in your heart that nobody can see.
Anyone that know God will never abide by the world i.e. a Christian would not listen to Worldly music, watch worldly movies, go clubbing, wear exposing dress, fornicate, bribe, cheat, steal, lie etc. and then say "Christianity is in the heart".

Further Reading: Mat 5:48, Gal 5:19-21

There are rooms for corrections and additions.

True. That's how we know that they don't talk to anyone. You can't talk to god and still disobey the same god? You dey craze?