A square is a rectangle but a rectangle is not necessarily a square

Rectangle - A plane figure with four straight sides and four right angles.
Square - A plane figure with four equal straight sides and four right angles.

You see that a square has one requirement that a rectangle does not have.  A square is a rectangle, but a rectangle is not necessarily a square.  A rectangle is only a square if all of its sides are of equal length.  So if I'm holding a square in my hand, you know it is correct to say I am holding a rectangle.  But if I tell you I am holding a rectangle in my hand, you would have to measure its sides to know whether that rectangle was in fact also a square.

What is the spiritual significance of this?  Read James 2:18-26,

18 But someone will say, “You have faith and I have works.” Show me your faith apart from your works, and I will show you my faith by my works. 19 You believe that God is one; you do well. Even the demons believe—and shudder! 20 Do you want to be shown, you foolish person, that faith apart from works is useless? 21 Was not Abraham our father justified by works when he offered up his son Isaac on the altar? 22 You see that faith was active along with his works, and faith was completed by his works; 23 and the Scripture was fulfilled that says, “Abraham believed God, and it was counted to him as righteousness”—and he was called a friend of God. 24 You see that a person is justified by works and not by faith alone. 25 And in the same way was not also Rahab the prostitute justified by works when she received the messengers and sent them out by another way? 26 For as the body apart from the spirit is dead, so also faith apart from works is dead.

Faith will produce good works but good works are not necessarily the product of faith.  Let me say it another way:  Good works are the product of faith; not the cause of it.  However, there are other things that cause good works.  Self-promotion can cause good works.  That's why Jesus said,

5 “And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward. 6 But when you pray, go into your room and shut the door and pray to your Father who is in secret. And your Father who sees in secret will reward you. (Matthew 6:5-6)

He said if we do things for the accolades of other people then we've already received our reward for doing them.  That is, no reward will come from God.  Self-esteem is another cause of good works.  From personal experience, sometimes I "do good" just so I feel better about myself and then I delude myself into thinking I'm better than someone who isn't as "good" as I am.

Of all the causes of good works only faith is credited to someone as righteousness.  In fact, at the judgment there will be people who have done good--maybe lots of it--and by doing so will think they are in right standing with God only to find out (to their great dismay) Jesus never knew them.  In Matthew 7:21-23 Jesus makes a frightening statement:

21 “Not everyone who says to me, ‘Lord, Lord,’ will enter the kingdom of heaven, but the one who does the will of my Father who is in heaven. 22 On that day many will say to me, ‘Lord, Lord, did we not prophesy in your name, and cast out demons in your name, and do many mighty works in your name?’ 23 And then will I declare to them, ‘I never knew you; depart from me, you workers of lawlessness.’

So you see the motivations behind your deeds is indeed critical.  But here's the thing, while this should produce a holy fear in you to examine your heart, when you look honestly at the condition of your heart I think you will see how corrupt your motivations are--not how pure they are.  For Psalm 14 says,

2 The Lord looks down from heaven
    on all mankind
to see if there are any who understand,
    any who seek God.
3 All have turned away, all have become corrupt;
    there is no one who does good,
    not even one.

When we truly understand how sinful we are (how much spiritual debt we've accrued), it takes us back to the cross where we remember our Savior died for us and forgave us of ALL of our sins (paid off ALL of our debt).  This will produce a harvest of joy and thanksgiving and out of the gratefulness of our hearts good works will come.

This is not a call to modify your behavior.  If you, like me, find that you just can't muster up the ability to love people who seem unlovable or serve people who need help understand the problem is not a lack of willpower--the problem is we've forgotten how much debt Christ paid off for us.  The process always starts with examining your heart and understanding your sinfulness.

So are you a square or a rectangle?  I hope this helps you see the harmony between the messages (faith and works) of the Bible.