1. "Why do you Christians continue to lie, backstab, gossip, malign and even oppress or bully others even after accepting Christ?"
I get this question in a myriad of paraphrases all too often.
Firstly, this is earth, not heaven. This is a truth so stark and plain, you'd be surprised how so many of us fail to get the point. Since Adam's fall, all humans' genes are already 'contaminated' with a sinful nature. We're all sinful by nature already. As Psalm 51:5 says, "Surely I was sinful at birth, sinful from the time my mother conceived me."
Becoming Christians doesn't mean our sinful DNA are suddenly stripped off and we're bestowed with a halo ring above our heads. Becoming Christians means we finally acknowledge 1 fact and commit to 1 thing. The 1 fact is that Christ is real and all His teachings are real and accurate. The 1 commitment is to love and obey the Lord with all our heart.
So Christians basically commit to obeying all of the Lord's commands (which includes not lying, backstabbing, gossiping, bullying, raping, robbing, etc) and loving the Lord and everyone else around, but because we're still stuck with our sinful DNA within us, we sometimes get tempted by Satan and choose to defy the Lord. It's just like how children sometimes wholeheartedly and sincerely promise not to be naughty anymore but as they're inherently playful, sometimes it just takes a little temptation and nudging to spark that naughtiness to act up.
2. "So why doesn't God just instantly remove Christians' sinful nature, so that you'd all become sinless? Why doesn't He just make Christians' life smooth-sailing and trouble-free since they've already acknowledged Him?"
As an excerpt of 2 Samuel 24:24 by King David goes "I will not sacrifice to the Lord my God burnt offerings that cost me nothing." I believe that sums up everything.
In the first place, God has already graciously sent His only Son, Jesus, to die on the cross for our sins, so that we don't have to be condemned in hell for our sins, so that all our sins are wiped out, as long as we confess all our sins and acknowledge that Jesus has helped wipe out those sins of ours.
Like diamonds, we need to be cut by other diamonds (as in, other humans) in order to be cut into that uniquely beautiful shape we were planned to be. Like a diamond-craftsman has a plan and specific shape he wants that diamond to be turned into, it's the same for God towards us - He has a unique specific plan for our lives, but we need to be moulded into that beauty through trials and cuttings.
The only difference is that diamonds don't get to choose, but we get to. Because God doesn't want to love robots that are programmed to love Him without choice. That kind of love, needless to say, is utterly meaningless.
He wants our reciprocal love and He wants us to be developed into a beautiful person, not by force, but with our willingness.
3. "But I think you Christians go to church and use it like a social club, networking club, or matchmaking agency."
Jesus advocates fellowship and love. He always go to people's houses to dine and fellowship, together with His disciples. There have been instances of Jesus dining, drinking, catching up with His friends as He also taught and enlightened them on many issues. That is healthy fellowship.
However, probably people have also seen some other people exploiting this healthy fellowship that was taught and encouraged.
Again, Christians are not saints. We commit to obeying His teachings and one of it being not to defile the church by making use of the church for the wrong means. Ask any Christian about the very rare occasion when Jesus actually got angry (righteous anger) and overturned tables in a temple because people were making use of the temple to do business. All of us have our own 'weak spots' when it comes to sin. Some of us are prone to lying, some of us are prone to gossip, while some of us are probably prone to profiteering (business, networking, partner-finding, finding gains, making use of others, etc.), even in the church.
Also, we all know that not everyone who comes to church is really interested in God, hence the occasionally outrageous business-making, networking, and partner-finding. Some come with ulterior motives or personal agendas. Even though this is a fact and we know it happens all around, we really shouldn't judge who are the ones doing that, because it's none of our business. Judging people is really God's business.
Even if people do come to the church with ulterior motives or personal agendas, and have no intention of actually committing their lives to God, they should still be welcomed and loved. Hear this from Matthew 9:12: "On hearing this, Jesus said, "It is not the healthy who need a doctor, but the sick.""
