Mark 7:7 (NKJV)
A substitute is something that is used in the place of another thing, something that is in some way equal to or the equivalent of something else. For example, if you are baking something you will need certain ingredients. But, it could be that you realize you are missing something you need. It may be that you have to have butter, or eggs, or something else that at the moment is not available. In this situation, you can often find an acceptable substitute to use in place of what you were supposed to have, and this substitute will work just as well as what you were originally going to use.
While there may be some acceptable substitutes for some baking ingredients, some things have no substitute; God's Word has no substitute, it has no equal. In Mark 7, Jesus pointed out a problem that the Pharisees had that many people still have today. Jesus told the Pharisees that what they were doing was in vain because they had created doctrines out of the commandments of men. The Pharisees had tried to substitute something else for the Word of God. Most Christians know about the Pharisees and the problems that they had, but they do not realize that they are often guilty of doing the same thing that the Pharisees did.
No matter what idea someone comes up with, no matter how good something may sound, or who does or does not do something, we can never substitute man's ideas for God's Word. We may not directly contradict what we find in the Bible with some man-made idea, but do we put more emphasis on our feelings, past experiences, something our favorite preacher said, or even a song than we do on God's Word? If we want success in life and to be able to take advantage of all of the blessings that God provided for us through Jesus, then the only way to do that is by knowing and applying the Word of God in our life, there is no acceptable substitute for God's Word!