What is the significance in believing in the Son? Is it not sufficient to believe in a loving and merciful Creator only?

There are several reasons that faith in Jesus Christ, the Son of God, is necessary for eternal life. I will mention several of them below:

1. It is only through the Son that we can truly know the Father. In Matthew 11:27All things are delivered unto me of my Father: and no man knoweth the Son, but the Father; neither knoweth any man the Father, save the Son, and he to whomsoever the Son will reveal him.
   See All..., Jesus said, "All things are delivered unto me of my Father: and no man knoweth the Son, but the Father; neither knoweth any man the Father, save the Son, and he to whomsoever the Son will reveal him." There are many false concepts of God. Men tend to emphasize the traits that most appeal to them and define God according to their own assumptions. However, although God has revealed Himself to us in various ways (creation, the Bible, His names, etc.), the most complete and accurate revelation of God the Father comes through the Son. Jesus plainly declared, "he that hath seen me hath seen the Father" (John 14:9Jesus saith unto him, Have I been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father; and how sayest thou then, Shew us the Father?
   See All...). Many people follow false concepts of God because them do not know the Son.
2. The Bible emphatically states the necessity of salvation through the Son Jesus Christ. Jesus declared, "Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me" (John 14:6Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me.
   See All...). After the resurrection and ascension of Jesus, Peter preached, "Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved" (Acts 4:12Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.
   See All...). It is through the Son that we come to know the Father.
3. God and man are so far apart in ability, moral nature, and in every other way that there must be someone to mediate between the two. This mediator must understand both God and man. Jesus Christ fulfilled this in that He was already God, being of the three-in-one with the Father, but He became man through his birth to Mary. He became the God-man. As such, He became the mediator between the two. 1 Timothy 2:5-6 [5] For there is one God, and one mediator between God and men, the man Christ Jesus; [6] Who gave himself a ransom for all, to be testified in due time.
   See All... states, "For there is one God, and one mediator between God and men, the man Christ Jesus; Who gave himself a ransom for all, to be testified in due time." He is able to bring God to man because He as God came down to man.
4. Jesus Christ provided the means by which we can come to know the Father. God, being absolutely holy, is so high that He can have no relation to man who is full of sin and defilement. There is no natural way by which these two coming from opposite poles can be brought together. We cannot simply believe in God. We cannot live a good enough life because even "all our righteousnesses are as filthy rags" (Isaiah 64:6But we are all as an unclean thing, and all our righteousnesses are as filthy rags; and we all do fade as a leaf; and our iniquities, like the wind, have taken us away.
   See All...) and are unacceptable to God. God, being absolutely just, requires that evil be judged. Certainly, we get our sense that wrongs ought to be paid for from God. Yet, our sins are so many and our ability to pay for them so weak that for us to pay for our own sins would require an eternal form of judgment. However, Jesus did more than just die a cruel death on the cross. He experienced God's wrath against our sin. He made a way for us to have a full cleansing of our sinfulness. We place our faith in the Lord Jesus Christ and we have His righteousness imputed to us. Concerning Christ, Paul desired to "be found in him, not having mine own righteousness, which is of the law, but that which is through the faith of Christ, the righteousness which is of God by faith" (Philippians 3:9Thy bow was made quite naked, according to the oaths of the tribes, even thy word. Selah. Thou didst cleave the earth with rivers.
   See All...).

So much more could be said, but I must limit the length of these answers. I hope and pray that this answer will help you see the necessity of the Son. One objection that may come to your mind might concern those who lived before the time of Christ. First, God was gracious toward their ignorance of the fullness of God. Acts 17:30And the times of this ignorance God winked at; but now commandeth all men every where to repent:
See All... states, "And the times of this ignorance God winked at; but now commandeth all men every where to repent." In addition, even those who believed in God in earlier ages were not allowed fully into the presence of Christ until the blood of Christ was applied to their souls. Jesus Christ is "the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance" (Hebrews 9:15And for this cause he is the mediator of the new testament, that by means of death, for the redemption of the transgressions that were under the first testament, they which are called might receive the promise of eternal inheritance.
See All...). The death of Christ was the means of eternal inheritance for those who were under the first testament (that is, those who brought the animal sacrifices in the Old Testament).

I pray that if you have not yet put your faith and trust in Jesus Christ, that you would allow God to work in your heart and trust in the Son as your eternal Saviour.