This world is a testing ground.

[TonyChanYT]

Why did God put the forbidden fruit in the middle of Eden?

To test the obedience and loyalty of Adam and Eve.

If our body in heaven will be sinless unlike our current fleshly body, why did god not just do that to begin with?

To test us. If we pass the test, we will go to heaven.

Why does God allow suffering and Satan to hang around?

2 Peter 3:

9 The Lord is not slow in keeping his promise, as some understand slowness. Instead he is patient with you, not wanting anyone to perish, but everyone to come to repentance.

Jesus told us the good news and the bad news in John 16:

33 “I have told you these things, so that in me you may have peace. In this world, you will have trouble. But take heart! I have overcome the world.”

God even makes use of the wicked, Proverbs 16:

4 The LORD has made everything for its purpose, even the wicked for the day of trouble.

Satan is the ruler of this world. This life that we are living is for testing to see who will get to inherit eternal life and heavenly rewards, James 1:

12 Blessed is a man who perseveres under trial; for once he has been approved, he will receive the crown of life which the Lord has promised to those who love Him.

Focus on God and love and the positives in life. Focus on your eternal rewards for the hardships that you have endured, Luke 16:

25 “But Abraham replied, ‘Son, remember that in your lifetime you received your good things, while Lazarus received bad things, but now he is comforted here and you are in agony.

In the end, God is just. We only live this earthly life once. Make the best of it, Revelation 22:

12 “Behold, I am coming quickly, and My reward is with Me, to render to every man according to what he has done.

Why do bad things happen to good people?

We don't know all the reasons, Isaiah 57:

1 The righteous perish, and no one takes it to heart; the devout are taken away, and no one understands that the righteous are taken away to be spared from evil.

Looking at it from the earthly perspective, there is no justice.
Looking at it from the eternal perspective, there is ultimate justice in God.

Does God test our faith?

Yes, 1 Peter 1:

7 so that the tested genuineness of your faith—more precious than gold that perishes though it is tested by fire—may be found to result in praise and glory and honor at the revelation of Jesus Christ.

Why does the Lord allow other religions to exist?

Why did God place the tree of knowledge of good and evil so prominently in the middle of the Garden of Eden (Genesis 2:9)?

To test people to see if they would choose to obey God or not.

This world is the testing ground. Those who pass will inherit the new earth. God permits wickedness in the world to test the hearts of man, good or bad so that the works of the godly might be displayed. If you enjoy wealth and health in this world, what are you doing with your possessions? Ultimately, everything leads to the Cross and the glory of the Cross.

See also Why did God make Satan?.