Christianity was supposed to be about becoming like Jesus.  
We went in the wrong direction.
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How can I know for sure I am saved?
 
Many people decide to become a Christian and find at first strong feelings. As time passes, these feelings may subside in intensity often leading people to ask if they are really saved.
What are our feelings for?
God gave us feelings to enjoy life and let us know when something is wrong. Satan has tried to create an artificial social environment where feelings are supposed to be relied upon to determine truth. If we come to use our feelings to measure our salvation, we can really get jerked around.

The heart is deceitful above all things, and desperately wicked: who can know it? - Jeremiah 17:9

I must be saved, I am not a bad person  
There is none that understandeth, there is none that seeketh after God. They are all gone out of the way, they are together become unprofitable; there is none that doeth good, no, not one. - Romans 3:11-12

For all have sinned, and come short of the glory of God; - Romans 3:23
I must be saved, look at the good I do
Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works? And then will I profess unto them, I never knew you: depart from me, ye that work iniquity. - Matthew 7:22-23

 

We can think some act we have done gave us salvation
Baptism 
Baptism is a public declaration that one identifies himself with the death burial and resurrection of Jesus. It does not cause salvation.

The idea that a baby can make this declaration is not Biblical. The idea that a baby can be saved or become a Christian by being sprinkled with water is a sad deception. 

 

  Church membership  
Many people feel that their membership with a particular denomination or faithful attendance at a particular church is a guarantee of salvation.
Saying a particular prayer
Some people have been "led" to Christ by being told to repeat a particular prayer or make a particular declaration. While often heartfelt, these prayers and declarations do not produce salvation.
Repenting of all known sin
Many people are told they have to "repent" or make Jesus "Lord" before they can be saved. In addition to leading people to be mistaken about the foundation of their faith, people can grow to be a little smug about how well they have repented and think they have justified themselves.
Making a decision
Many people have "come forward" in an alter call or at a crusade meeting. These people may think that because they have made the decision to come forward, that they are now saved.

 

 
The Bible says both that we can "know" we are saved (1John 5:13), and that we should examine ourselves to see if we are in the faith (2 Corinthians 13:5). This is not a contradiction. The Bible tells us that there is uncertainty and can be certainty. The Bible also tells us to persevere. To the theologian this apparent ambiguity is something he must resolve and explain. As a result, there are many doctrines of various denominations that are promulgated more for their tidiness than their accuracy.

We might want to consider that the lack of Biblical specificity is not an error, but rather the desire of God that we derive our assurance of salvation directly from Him. God may want us unsure and ill at ease until we mature in faith and grow in the image of Christ and closer to Him.

Some religious experts offer measurement standards such as having been baptized, saying a particular prayer, having taken Jesus "into your heart", or made Jesus Lord of your life as the criteria to guarantee that one is saved.

 

If trusting is all that is needed for salvation, how can I know if I have trusted enough?
And this is that testimony (that evidence): God gave us eternal life, and this life is in His Son. He who possesses the Son has that life; he who does not possess the Son of God does not have that life. I write this to you who believe in (adhere to, trust in, and rely on) the name of the Son of God [in the peculiar services and blessings conferred by Him on men], so that you may know [with settled and absolute knowledge] that you [already] have life, yes, eternal life. - 1 John 5:11-13 (Amplified)

Rom 8:16 The Spirit Himself [thus] testifies together with our own spirit, [assuring us] that we are children of God.

In Him you also who have heard the Word of Truth, the glad tidings (Gospel) of your salvation, and have believed in and adhered to and relied on Him, were stamped with the seal of the long-promised Holy Spirit. That [Spirit] is the guarantee of our inheritance [the firstfruits, the pledge and foretaste, the down payment on our heritage], in anticipation of its full redemption and our acquiring [complete] possession of it--to the praise of His glory. - Ephesians 1:13-14 (Amplified)

  What if I am still uncertain?  
Wherefore the rather, brethren, give diligence to make your calling and election sure: for if ye do these things, ye shall never fall: - 2 Peter 1:10

Examine yourselves, whether ye be in the faith; prove your own selves. Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? - 2 Corinthians 13:5
Biblical tests
For the preaching of the cross is to them that perish foolishness; but unto us which are saved it is the power of God. - 1 Corinthians 1:18 (Do you think the gospel is foolish?)

We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death. - 1 John 3:14 (Do you have love for other Christians?)

Joh 18:37 Pilate therefore said unto him, Art thou a king then? Jesus answered, Thou sayest that I am a king. To this end was I born, and for this cause came I into the world, that I should bear witness unto the truth. Every one that is of the truth heareth my voice. - John 8:37 (Are you "of the truth"?)

 

Two paths
And beside this, giving all diligence, add to your faith virtue; and to virtue knowledge; And to knowledge temperance; and to temperance patience; and to patience godliness; And to godliness brotherly kindness; and to brotherly kindness charity. For if these things be in you, and abound, they make you that ye shall neither be barren nor unfruitful in the knowledge of our Lord Jesus Christ. But he that lacketh these things is blind, and cannot see afar off, and hath forgotten that he was purged from his old sins. - 2 Peter 1:5-9

The Christian for any one of a number of reasons can end up far from his Lord. The key to assurance of salvation is closeness to Him who saved us.

  How do we come close to Jesus?  
But he giveth more grace. Wherefore he saith, God resisteth the proud, but giveth grace unto the humble. Submit yourselves therefore to God. Resist the devil, and he will flee from you. Draw nigh to God, and he will draw nigh to you. Cleanse your hands, ye sinners; and purify your hearts, ye double minded. Be afflicted, and mourn, and weep: let your laughter be turned to mourning, and your joy to heaviness.  Humble yourselves in the sight of the Lord, and he shall lift you up. - James 4:6-10

The process James describes starts and ends with humility. 
Why do we have difficulty discerning the Spirit?
And that he died for all, that they which live should not henceforth live unto themselves, but unto him which died for them, and rose again. - 2 Corinthians 5:15

For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would. - Galatians 5:17

Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him. For all that is in the world, the lust of the flesh, and the lust of the eyes, and the pride of life, is not of the Father, but is of the world. - 1 John 2:15-16