Pay Your Vows 
Hear, O my people, and I will speak; O Israel, and I will testify against thee: I am God, even thy God. I will not reprove thee for thy sacrifices or thy burnt offerings, to have been continually before me. I will take no bullock out of thy house, nor he goats out of thy folds. For every beast of the forest is mine, and the cattle upon a thousand hills. I know all the fowls of the mountains: and the wild beasts of the field are mine. If I were hungry, I would not tell thee: for the world is mine, and the fulness thereof. Will I eat the flesh of bulls, or drink the blood of goats? Offer unto God thanksgiving; and pay thy vows unto the most High: And call upon me in the day of trouble: I will deliver thee, and thou shalt glorify me. 
(Psalms 50:7-15)
The Bible is full of prophesies, in fact it is the one book above all books of other religions that contains prophesies. And what is more, many of the prophesies have already come to pass, proving beyond all possible doubt that it is the word of God. 

A prophecy is nothing else but a word from God that He placed in a servant's heart to be recorded for future peoples to acknowledge. 
However, not all words from God concern future happenings and this one in particular is a very clear word from God explaining what He expects from mankind. 
From studying Heart of the Gospel we know that accepting Jesus Christ as our Saviour is nothing less than making a Covenant with God through the Blood Sacrifice of mankind's representative, Jesus of Nazareth. 

Further, it has repeatedly been explained in these writings that God expects certain actions on our part when we accept Jesus Christ as our Lord and Master. 
In the passage above, we have conclusive confirmation that if we are obedient to the vows of the Covenant, we shall be protected in the day of trouble. 
Many professing Christians stand on the promises of Psalm 91 and are comforted thereby, but do verses one and nine apply to them ? They accept the privileges of Covenant, but are they responsible to the conditions of Covenant? 
O Lord, open thou my lips; and my mouth shall shew forth thy praise. For thou desirest not sacrifice; else would I give it: thou delightest not in burnt offering. The sacrifices of God are a broken spirit: a broken and a contrite heart, O God, thou wilt not despise. 
(Psalms 51:15-17)
He that dwelleth in the secret place of the most High shall abide under the shadow of the Almighty. I will say of the LORD, He is my refuge and my fortress: my God; in him will I trust. Surely he shall deliver thee from the snare of the fowler, and from the noisome pestilence. He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler. Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day; Nor for the pestilence that walketh in darkness; nor for the destruction that wasteth at noonday. A thousand shall fall at thy side, and ten thousand at thy right hand; but it shall not come nigh thee. Only with thine eyes shalt thou behold and see the reward of the wicked. Because thou hast made the LORD, which is my refuge, even the most High, thy habitation; There shall no evil befall thee, neither shall any plague come nigh thy dwelling. For he shall give his angels charge over thee, to keep thee in all thy ways. They shall bear thee up in their hands, lest thou dash thy foot against a stone. Thou shalt tread upon the lion and adder: the young lion and the dragon shalt thou trample under feet. Because he hath set his love upon me, therefore will I deliver him: I will set him on high, because he hath known my name. He shall call upon me, and I will answer him: I will be with him in trouble; I will deliver him, and honour him. With long life will I satisfy him, and shew him my salvation. 
(Psalms 91:1-16)
Let's now add the last part of the first passage above
Offer unto God thanksgiving; and pay thy vows unto the most High: And call upon me in the day of trouble: I will deliver thee, and thou shalt glorify me. But unto the wicked God saith, What hast thou to do to declare my statutes, or that thou shouldest take my covenant in thy mouth? Seeing thou hatest instruction, and castest my words behind thee. 
(Psalms 50:14-17)
My friend, if you are still reading this and are disturbed in your soul about these words, this is good. It shows that God the Holy Spirit is still working in your life, leading you on the paths of righteousness for His name's sake. 
Please take God's advice - He loves you so.

Come and hear, all ye that fear God, and I will declare what he hath done for my soul. I cried unto him with my mouth, and he was extolled with my tongue. If I regard iniquity in my heart, the Lord will not hear me: But verily God hath heard me; he hath attended to the voice of my prayer. Blessed be God, which hath not turned away my prayer, nor his mercy from me.  

(Psalms 66:16-20)