Earlier that evening Jesus had declared that one of them would betray Him, one would deny Him, and all of them would stumble (cf. Mark 14:20ff). Also, Jesus had foretold that He would not be with them much longer (cf. John 13:33). Certainly these words were greatly disturbing to the apostles and would have caused much mental distress and discouragement. Nevertheless, He does not want their hearts to be troubled.
"You believe in God, believe also in Me" - Jesus strives to encourage them by appealing to their faith and trust in the Father. Their faith in God, and the understanding that He loves them and knows what is best, would provide strength. There is some ambiguity here regarding the translation of this part of the verse. Some believe that a better translation would be: "Believe in God, believe also in Me." If this is correct, it is easy to see that the meaning is slightly changed (i.e., the emphasis would be more on their need to have faith in the Godhead in general, in contrast to emphasizing the need for faith in Jesus as a consequence of faith in God).
Jesus, in this lengthy discourse recorded in John 14-16, seeks to calm the anxiety of His disciples and better prepare them for His departure. They are going to need all of the edification they can get since they are about to see Him whom they love slandered, abused, condemned, and slain. Their desires and expectations of an earthly kingdom would die right along with Jesus.
Today, when we are in the midst of severe trials, let us remember this verse. Let us hold on tight to our faith and trust in the Godhead and not allow anxieties to find a nesting place in our hearts.
It needs to be remembered that in this context (i.e., John 14-16), Jesus is with the eleven apostles (Judas has already departed). Thus, one must be careful how he interprets these verses. One must not automatically assume that everything Jesus says herein will directly apply to disciples of the 21st century. For example, John 14:26 and 16:13,14 are specific promises to the apostles, not Christians today (cf. Jude 3).
"In My Father's house are many mansions...I go to prepare a place for you" - The disciples would also find comfort (as we do today) in the fact that a heavenly home awaited them. They would not always be "homeless," as they were then. Jesus spoke the truth, as always (cf. Titus 1:2), and assured them that there would be plenty of room in heaven for the faithful (cf. Luke 14:22). One of the reasons for His departure was so He could continue preparing that heavenly abode (cf. Matt. 25:34). Although Jesus' death would provide mankind with the fountain of cleansing from sin (cf. Zech. 13:1), and thereby open the way to heaven, there were still some heavenly preparations that needed to be made.
It should be noted that the word for "mansions" can literally be translated as abiding-places. So, whether our resting places in heaven are like earthly mansions or not is unknown (and, to be blunt, inconsequential). Regardless of what the abiding places in heaven look like, certainly they will be wonderful in every way!
We will continue studying this narrative in our next lesson.