"Boast not thyself of to morrow; for thou knowest not what a day may bring forth." (Proverbs 27: 1)
Many read this verse, and conclude that tomorrow is a mystery, and they can have no assurance about tomorrow.
Let us look at the verse and let the Spirit of God minister to us through the Word of God:
"It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, they are spirit, and they are life." (John 6: 63)
"31Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed; 32And ye shall know the truth, and the truth shall make you free." (John 8: 31-32)
The first word of Proverbs 27 is "boast", in the original Hebrew "halal", the same root for Lucifer:
"How art thou fallen from heaven, O Lucifer (Helel), son of the morning! how art thou cut down to the ground, which didst weaken the nations!" (Isaiah 14: 12)
Lucifer, later Satan, was the chief angel, who held two offices, yet rebelled against God's authority, and tried to make himself God, stating "I will" as recorded in Isaiah 14.
The verse speaks about boasting of ourselves about tomorrow, or praising ourselves for the things that we are going to accomplish in the next day or days.
The issue is not so much the mystery of the next day, since Jesus Himself encourages us not to take thought for the morrow (Matthew 6: 33-34).
This proverb actually dissuades us from taking pride in our own strength, for we do not know what a day may bring forth or "give birth to " in the Original Hebrew.
Yet God has brought forth many things for us, in one day.
Keep in mind that the world would never have conceived of God becoming a man, and yet Jesus came:
"Therefore the Lord himself shall give you a sign; Behold, a virgin shall conceive, and bear a son, and shall call his name Immanuel." (Isaiah 7: 14)
"How long wilt thou go about, O thou backsliding daughter? for the LORD hath created a new thing in the earth, A woman shall compass a man." (Jeremiah 31: 22)
What else was brought forth in a day? The nation of Israel!
"Who hath heard such a thing? who hath seen such things? Shall the earth be made to bring forth in one day? or shall a nation be born at once? for as soon as Zion travailed, she brought forth her children." (Isaiah 66: 8)
This verse also speaks of the Body of Christ, as we were born again in Christ, and we are His body.
We do not boast in ourselves for tomorrow because we have no strength in ourselves:
"I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing." (John 15: 5)
We may not know what the future holds, but we need who holds the future:
"Declaring the end from the beginning, and from ancient times the things that are not yet done, saying, My counsel shall stand, and I will do all my pleasure:" (Isaiah 46: 10)
"He stretcheth out the north over the empty place, and hangeth the earth upon nothing." (Job 26 7)
God who is outside of time, who declares the end from the beginning, who hangs the world on nothing, cares for you and me (1 Peter 5: 6-7), and has nothing but good planned for us:
"For I know the thoughts that I think toward you, saith the LORD, thoughts of peace, and not of evil, to give you an expected end." (Jeremiah 29: 11)
"Now our Lord Jesus Christ himself, and God, even our Father, which hath loved us, and hath given us everlasting consolation and good hope through grace," (2 Thessalonians 2: 16)
We do not bring forth anything in tomorrow, but God our Father does, and we can trust that He has nothing but good planned for us. Let hope (the confident expectation of good) be your helmet today (1 Thessalonians 5: 8) and trust Him for your tomorrows.