Is positive confession or to declare and decree biblical?
When God speaks, things happen. Even the gospel itself has the power of God in it (Rom 1:16; 1 Cor 1:18), so when God speaks, His Word shall be fulfilled. God says, “so shall my word be that goes out from my mouth; it shall not return to me empty, but it shall accomplish that which I purpose, and shall succeed in the thing for which I sent it” (Isaiah 55:11). No one or nothing can stop it from accomplishing His purpose. Scripture teaches that God “spoke, and it came to be; he commanded, and it stood firm” (Psalm 33:9). We can speak commands or for something to stand firm, but we don’t have the power to bring it about like God can. We are out of our league when we declare that something that doesn’t yet exist will come into existence, or when we decree something to be, when we have no power to bring it about. We can tell people we are declared righteous before God, but we certainly didn’t bring it to pass (2 Cor 5:21). Declarations are not causative. I can say 2 plus 2 is 5 but I cannot cause it to be so, however, in some Christian circles, believers declare and decree over many things, “calling those things which are not, as though they were” (Rom 4:17), but only God Himself can say such a thing. That is an attribute of God, not man. God cannot lie, but “every man a liar” (Rom 3:4). Only He can cause “those things which are not, as though they were.” 
Declaring and decreeing is what we call positive confession. It’s someone declaring that something will be and decreeing that it will come to pass, but that strikes me a bit odd because humans have no power in words to bring things into being which now do not exist. We cannot change people and things just by speaking change into them. For God, that’s nothing difficult, but we cannot even guarantee a promise because circumstances come up and things change, so we might not be able to keep that promise, but a promise from God is as good as done. It will come to pass, just as Jesus promised He would return again (Acts 1:10-11; Rev 1:7). The Apostle John said that “this is the confidence that we have toward him, that if we ask anything according to his will he hears us” (1 John 5:14), but we must realize it must be “according to his will” and not ours. God’s will is better than ours since we don’t always know what’s best for us. God always does since He alone can see around the corners of time, and in fact, He’s been there and back. Jesus is God and He said, “Heaven and earth will pass away, but my words will not pass away” (Matt 24:35).
There are differences between declaring and decreeing something. If we declare something, we say something in a solemn manner, to make something publically known, to declare a truth, or even declare a national holiday, but in the latter case, we don’t have the power or authority to declare a national holiday. That takes an act of Congress. We can declare it, but it may or may not come true. God can declare the end from the beginning, as He has in His Word (Gen 1:1-Rev 22:21). God says, “remember the former things of old; for I am God, and there is no other; I am God, and there is none like me” (Isaiah 46:9). Only the Great I AM can be “declaring the end from the beginning and from ancient times things not yet done” (Isaiah 46:10a). Why? God answers, “My counsel shall stand, and I will accomplish all my purpose” (Isaiah 46:10b). Our counsel and our plans may or may not come to fruition, but God alone can declare, “I will accomplish all my purpose.” We dare not encroach on this holy ground of God’s sovereignty. Yes, we can receive what we pray for, but it must be according to His will, and not our declaring or decreeing it to be. We can’t declare, decree, or pray for our will in heaven as it is on earth, but His will on earth as it is in heaven.
To decree something is a lot more powerful than declaring something. To decree something generally means to make an official order which is issued by a legal authority; a rule of law usually issued by a head of state, so it has the force of law behind it. Only God can decree something. Something like the immutability of God or His Word is decreed. One example of God decreeing something was during the times when Israel and Judah were falling into idolatry. Jeremiah the Prophet rebuked these nations and spoke through Jeremiah, saying, “The Lord of hosts, who planted you, has decreed disaster against you, because of the evil that the house of Israel and the house of Judah have done, provoking me to anger by making offerings to Baal” (Jer 11:17). God can decree such things (anything, in fact!) because He is God and He knows the end of things from the beginning; He knows those things which are not yet as though they already are. We cannot decree things for God or for people to do…or declare God will do things for us or others. We must always pray for the will of God.
If we declare anything, it should be the gospel of repentance and faith in Jesus Christ (Mark 1:15). To decree something, I’d have to be in charge somewhere and over some people, but God is in charge of the universe and not me (thankfully!). His Word shall come to pass. God’s Word and God’s promises are irrevocable. God has declared and decreed that someday that “He will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away” (Rev 21:4). We do not have the authority to declare and decree. We have no authority from God to declare things in other people’s lives when only God knows their future. I’ve had many declare and decree things over me that never came to pass…but 100% of what God declares and decrees does! Christians must avoid declaring things for and in a person’s life and start declaring the gospel of our Lord and Savior, Jesus Christ, Who commands everyone to repent (Acts 17:30).
Here is some related reading for you: Are There Still Prophets Today? 
Resource – Scripture quotations are from The Holy Bible, English Standard Version® (ESV®), Crossway Bibles. (2007). ESV: Study Bible: English standard version. Wheaton, Ill: Crossway Bibles. Used by permission. All rights reserved.