John 14:1--3

"Let not your heart be troubled; believe in God, believe also in Me.  In My Father's house are many dwelling places; if it were not so, I would have told you.  And if I go and prepare a place for you, I will come again, and receive you to Myself; that where I am, there you may be also."

Through years of watching and listening to the Master, His chosen men had often been awed by His actions, intimidated by His power, worried over His waning popularity... but now He is talking of leaving them, and their hearts are troubled.
He had been their shield in so many ways...calming a storm that threatened to capsize their boat... defending them against accusations and attacks from the religious leaders, such as when the disciples picked grain on the Sabbath... even diffusing the anger of mobs intent upon killing Him (and them)...  So, now that He is talking of leaving them, their hearts are troubled.
He had also been their spiritual Protector, as well.  At one point, He revealed to Peter that "Satan has demanded permission to sift you like wheat, but I have prayed for you, that your faith may not fail," (Luke 22:31, 32a).  Such, indeed, was one of His greatest means of spiritual protection on their behalf:  intercessory prayer (John 17:6--19).  The other was the sheer force of who He was (John 18:6).
And His presence was now being taken from them... their hearts were troubled, indeed.

Our Lord's first comforting assurance dealt with His divinity.  "believe in God, believe also in Me."  In truth, it was at the heart of His relationship to them.  Their total acceptance of His equality with the Father was a non-negotiable, the very foundation under-girding what He was about to promise.  They had to embrace who He was, the God-Man, in order to trust Him (as He knew they would) in the future.  Psalm 9:10 speaks to this:  "And those who know Your Name will put their trust in You; For You, O LORD, have not forsaken those who seek You."  Those trust God the most who know Him the best.  The reason we do not trust Him more is because we do not know Him, really.  The more intentional we are in hearing, reading, studying, memorizing & meditating on His Word, the deeper will be our knowledge of Him.  And the more faithful we find Him, the greater will be our confidence in all that He promises.
It was so that evening in the upper room... the Lord Jesus sought to comfort their troubled hearts by revealing (1) heaven's accommodations, (2) His provision there for His own, and (3) His imminent death on the cross would only be a temporary separation from them-- there would be an eternity-long enjoying of His presence in glory.

(1) Better than "mansions," "dwelling places" (or apartments) conveys the layout our Lord is describing.  The disciples were no doubt familiar with the practice of adding rooms to the main house when children would marry, elderly relatives would need a place to spend closing years, or for extending hospitality to visitors.  How many "dwelling places" the father could add on was, of course, limited by his financial resources.  In contrast, heaven has no such restriction... limited only in the number of the elect ones who will reside there.
(2) The Lord Jesus Himself will make the needed preparations... matching each deceased (or translated) saint with a dwelling place suitable for his/her glorified existence.  Such a loving Lord!  Numberless angels could have been sent for this task, but no... the sovereign Shepherd Himself will tend to His sheep (John 10:4).  From eternity past when we were chosen in Him, through our days on this earth as we are led by His sent Holy Spirit, and into a heavenly eternity... we have been graciously provided for by our faithful Savior.
(3) His promised return was based on the pledge of preparing their (and our) heavenly home.  "I will come again" is the confident hope that everyone who has been changed by the Lord Jesus holds tenaciously.  
Note:  our Lord did NOT say that He would come again and take us to heaven...rather,  "I will come again and receive you to Myself.."  In a wonderful sense, going to heaven is not so much going to a place, as it is going to a Person.  Heaven is being with Him, forever!    

"...if it were not so, I would have told you."  With their winsome wooing, these assurances by the Savior draw my heart back time and again... for, who among us hasn't had a troubling thought about death and its hidden coming..?  The enemy of our soul knows only too well the tendency we have, as people, to dread the unknown, to fear the unseen, and to doubt what is beyond our senses.  How gracious and compassionate of the Lord Jesus to give us the written Word of His immutable promises of: His preparations for us, His provision, and His Person.  Such a joy it will be to praise Him more worthily than we do now... and forever, at that!