Unequally Yoked-Believer's & Non Believer's Together

If you are a follower of Christ is it okay to hang around with non-believer's? If you are planning to get married is it okay to marry someone who does not believe that Christ died on the cross for our sins? What if you find out after you are married that your spouse or significant other does not believe in God?
When Paul was speaking to the church's of Corinth he addressed the subject of believers associating with unbelievers saying that we as Christians should not be yoked (tied together) with unbelievers and he pointed out the most obvious reasons that there would be no common ground between them, clearly stating that good and evil do not belong together.
Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? I Cor. 6:14
There is no agreement between believers and unbelievers
What accord has Christ with Belial? Or what portion does a believer share with an unbeliever? What agreement has the temple of God with idols? For we are the temple of the living God; as God said, “I will make my dwelling among them and walk among them, and I will be their God, and they shall be my people. Therefore go out from their midst, and be separate from them, says the Lord, and touch no unclean thing; then I will welcome you, and I will be a father to you, and you shall be sons and daughters to me, says the Lord Almighty.  2 Cor. 6:14-18
Do Not be unequally yoked with unbelievers
Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? What accord has Christ with Belial? Or what portion does a believer share with an unbeliever? What agreement has the temple of God with idols? For we are the temple of the living God; as God said, “I will make my dwelling among them and walk among them, and I will be their God, and they shall be my people. Therefore go out from their midst, and be separate from them, says the Lord, and touch no unclean thing; then I will welcome you, and I will be a father to you, and you shall be sons and daughters to me, says the Lord Almighty.”2 Corinthians 6:14-18
Watch out for people who appear to be Christians that do not admit the Lord has power.
Having the appearance of godliness, but denying its power. Avoid such people. 2 Tim. 3:5
Marriage between believer's and non believers
To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her. If any woman has a husband who is an unbeliever, and he consents to live with her, she should not divorce him. For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy. But if the unbelieving partner separates, let it be so. In such cases the brother or sister is not enslaved. God has called you to peace. I Cor. 7:12-15 ESV
Warning not to marry unbeliever's
You shall not intermarry with them, giving your daughters to their sons or taking their daughters for your sons, for they would turn away your sons from following me, to serve other gods. Then the anger of the Lord would be kindled against you, and he would destroy you quickly. Deut. 7:3-4
Be Careful while you are testifying
While it is our duty to spread the word of the Lord to unbelievers, we must be extra careful not to imitate or do wrong things just to get in good with them so you can tell them about Jesus because it won't work-one slip and you will be right there with them. We are to testify but not imitate un-Christ-like behavior. We are told to stay away from those who are lazy and do not follow Christ's example.
Now we command you, brothers, in the name of our Lord Jesus Christ, that you keep away from any brother who is walking in idleness and not in accord with the tradition that you received from us. 2 Thessalonians 3:6
We are to expose evil
Take no part in the unfruitful works of darkness, but instead expose them. Eph. 5:11 ESV
Do not pass judgement on one another or on those who are weaker in their faith than you.
As for the one who is weak in faith, welcome him, but not to quarrel over opinions. One person believes he may eat anything, while the weak person eats only vegetables. Let not the one who eats despise the one who abstains, and let not the one who abstains pass judgment on the one who eats, for God has welcomed him. Who are you to pass judgment on the servant of another? It is before his own master that he stands or falls. And he will be upheld, for the Lord is able to make him stand. One person esteems one day as better than another, while another esteems all days alike. Each one should be fully convinced in his own mind. ...Romans 14: 1:23
How will you know if someone is not a believer? 
You will recognize them by their fruits or in other words by what they are producing or doing.
You will recognize them by their fruits. Are grapes gathered from thorn bushes, or figs from thistles? So, every healthy tree bears good fruit, but the diseased tree bears bad fruit. A healthy tree cannot bear bad fruit, nor can a diseased tree bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus you will recognize them by their fruits. Matt. 7:16:20
The Lord said not to touch unclean things if we plan to be welcomed by Him
Therefore go out from their midst, and be separate from them, says the Lord, and touch no unclean thing; then I will welcome you, 2 Cor. 6:17 ESV
We are to imitate good
As Christians we should always strive to become more Christ-like. If we imitate good we are from God and when we imitate evil-we will not see God or even be allowed in His Holy presence.
Beloved, do not imitate evil but imitate good. Whoever does good is from God; whoever does evil has not seen God. 3 John 1:11 ESV
If we do not follow Christ's example we do not have God 
Everyone who goes on ahead and does not abide in the teaching of Christ, does not have God. Whoever abides in the teaching has both the Father and the Son. 2 John 1:9 ESV
​Do not be unequally yoked with unbelievers. For what partnership has righteousness with lawlessness? Or what fellowship has light with darkness? I Cor. 6:14