The "chosen testimony" is referring to His disciples that abide in him and His words to be fruitful.
John 15:1I am the true vine, and my Father is the husbandman. 2 Every branch in me that beareth not fruit he taketh away: and every branch that beareth fruit, he purgeth it, that it may bring forth more fruit. 3 Now ye are clean through the word which I have spoken unto you. 4 Abide in me, and I in you. As the branch cannot bear fruit of itself, except it abide in the vine; no more can ye, except ye abide in me. 5 I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. 6 If a man abide not in me, he is cast forth as a branch, and is withered; and men gather them, and cast them into the fire, and they are burned. 7 If ye abide in me, and my words abide in you, ye shall ask what ye will, and it shall be done unto you. 8 Herein is my Father glorified, that ye bear much fruit; so shall ye be my disciples.
16 Ye have not chosen me, but I have chosen you, and ordained you, that ye should go and bring forth fruit, and that your fruit should remain: that whatsoever ye shall ask of the Father in my name, he may give it you.
As it is; not every saved believer will continue to abide in Him & His words as His disciples; the chosen, as the elect.
John 8:30 As he spake these words, many believed on him. 31 Then said Jesus to those Jews which believed on him, If ye continue in my word, then are ye my disciples indeed; 32 And ye shall know the truth, and the truth shall make you free.
It is not a loss of salvation when those left behind are still saved and thus still His servants, but as many are called, few are chosen.
Fortunately, for those wayward saints left behind at the rapture event, they can still love Him since He first loved us.