If you were not an eyewitness of the event, you are not absolutely sure that the event happened. The next day several eyewitnesses may come up to you to tell you that it happened, and each additional witness can strengthen your belief (faith) that the event occurred.
One’s faith/belief [pistis] can be “strengthened”
“But behold, as the seed swelleth, and sprouteth, and beginneth to grow, then you must needs say that the seed is good; for behold it swelleth, and sprouteth, and beginneth to grow. And now, behold, will not this strengthen your faith? Yea, it will strengthen your faith: for ye will say I know that this is a good seed; for behold it sprouteth and beginneth to grow” (Alma 32:30).
“Now they did not suppose that salvation came by the law of Moses; but the law of Moses did serve to strengthen their faith in Christ; and thus they did retain a hope through faith, unto eternal salvation, relying upon the spirit of prophecy, which spake of those things to come” (Alma 25:16).
One’s testimony/witness [martyria] can be “added upon”
“And they said, What need we any further witness [martyria]? for we ourselves have heard of his own mouth” (Luke 22:71).
“Verily, verily, I say unto thee, We speak that we do know, and testify [martyreō] that we have seen; and ye receive not our witness [martyria]” (John 3:11).
Revelation 19:10 states that “the testimony [martyria] of Jesus is the spirit of prophecy.”
Strictly speaking, one’s testimony/witness of a person or event is not “strengthened,” since it is first-hand knowledge that is not swayed in any direction by additional information.
However, if one personally defines a testimony as a total summation of things that he or she knows or has witnessed, then a testimony can “grow,” “be enlarged,” or “be added upon.” So loosely speaking, if one defines a “strong” testimony as one that contains more pieces of knowledge, then a testimony can be “strengthened.”
“The law of the Lord is perfect, converting the soul: the testimony ['eduwth] of the Lord is sure ['aman], making wise the simple” (Psalms 19:7).
I have a testimony that Jesus Christ lives.